In the following excerpt from Sedgewick (one of the WCF divines), we see a reference to a hypothetical covenant. In this section, Sedgewick does not condemn the doctrine, nor specifically endorse it. In the Tables, though, he says:

There is an absolute Covenant,
And an Hypothetical Covenant,

From my searching of this work so far, it is not apparent that Sedgewick later develops this idea in this work.

Sedgewick:

Of the Covenant in special.

I shall now descend to something more special, to show unto you, what that Covenant which God makes between himself and his people.

There are those who distinguish of a twofold Covenant.

1. There is Foedus absolutum, which is such a promise of God, as takes in no stipulation or condition at all, that runs altogether upon absolute terms; such a Covenant was that which God made with Noah, that he would never down the world any more. Gen 9.11. and such a kind of Covenant is that, when God promises to give faith and perseverance unto his elect, Heb. 8.10, &c. Both these Covenants are absolute, and without any condition; there is nothing in them but what is folded up in the promises themselves.

2. Foedus Hypotheticum, which is a gracious promise on God’s part, with an obligation to duty; for although it be natural to God, to recompense any good, as it is to punish any evil; And although man does owe unto God whatsoever God covenants with him for; yet it so pleases his Divine Will thus to deal with us, that in binding of us to duty unto himself, he binds himself in reward unto us, and promises such and such a recompence, upon the condition of such and such a performance.

Obadiah Sedgwick, The Bowels of Tender Mercy Sealed in the Everlasting Covenant (Printed by Edmund Mottershed, for Adoniram Byfield, and are to be sold by Joseph Cranford, at the Sign of the Castle and Lyon in St. Pauls Church-yard, 1661), 6. [I should note, that by posting this comment from Sedgewick, I am not suggesting that Sedgewick was an Amyraldian or even sympathetic to hypothetical universalism.]

Preston:

1) Now, for the opening Point to you, you must understand, that there are two ways, or Covenants, whereby God offers salvation to men: One, is the Covenant of Works, and that was that righteousness by which Adam had been saved, if he had stood in his innocency; for it was that way that God appointed for him, Do this, and live: But Adam performed not condition of that Covenant, and therefore now there is another Covenant, that is, the Covenant of Grace; a board given against Shipwreck. Now this covenant of Grace is double:

Either absolute and peculiar,

Or conditional.

Absolute and peculiar only to the elect. So it is expressed, Jer 31. I will put my law in your inward parts, and write in your hearts, and I will be your God, and you shall be my people. So likewise in Ezech. 36. I will give you a new heart, and put a new spirit within you, and I will take your stony hearts out of your bodies. Here the Covenant is expressed absolutely, and this is proper only to the Elect.

But now, besides, there is a conditional Covenant of Grace, which is common to all; and that is expressed in these terms: Christ has provided a Righteousness, and Salvation; that his Work that he has done already. Now if you will believe, and take him upon those terms that is offered, you shall be saved. This I say belongs to all men. This you have thus expressed in the Gospel many places: If you believe, you shall be saved: as it is Mark 16. Go preach the Gospel to every creature under Heaven; he that will believe, shall be saved; and he that will not believe, shall be damned. It is the same with that, Rom. 4.5 To him which works not, his faith is accounted righteousness. (Mark it) To him that believes on him that justifies the ungodly; that is; there is a certain justice of righteousness, that Christ has prepared or purchased for men, though they be ungodly, he requires nothing of them before-hand, though they be wicked and ungodly, yet this righteousness is prepared for them: that which is required of them, is only that they take it.   John Preston, The Breast-Plate of faith and Love, (London: Printed by George Purstow, and are to be sold in the Companie of the Stationers, 1651), 30-33.

2) Object. 6.

It is objected that the Covenant that is made by God seems to be made with the elect only, and therefore the condition belongs only to them, how can Christ belong to all, seeing the exhortation and commandant must not exceed the Covenant, for the benefit is propounded to the Elect, and the condition to be required of none but of such as are within the covenant.

Answ.

To this I answer, that there is a Covenant of grace, and that is double, either a general covenant propounded without exception, “Let whosoever will come and believe in Christ shall be saved,” here is none excluded, and that none are excluded out of this general covenant this reason will show. Baptism the seal of the Covenant is to be administered to all within the Church, to infants though afterwards they do not actually and visibly believe. Now God would not appoint that the seal of the Covenant should be given to those whom it does not generally belong. But secondly, there is another Covenant of grace which belong peculiarly to the elect, for in this God does not only promise to give salvation, if men believed, but he promises to give them ability to believe, as may be seen, Jer, 31:33, Ezekiel 36. In the first place God promises that he will “put his law in their inward parts, and that they shall not teach any more every man his neighbor, but they shall all know me from the least to the greatest,” and so the life in other places, where God promises the thing, and ability of performing it, which belongs only to the elect. But the other general Covenant belongs to all without exception.  John Preston, Riches of Mercy to Men in Misery (London: Printed by J.T. and are to be sold by John Alen at the rising sun in Saint Pauls Church Yard),  425.  [Italics original, some spelling modernized, marginal references and headers cited inline, and underlining mine.]

1) –In and for itself then it was the entire fallen race of man which God made the object of His gracious revelation, when He resolved to present to humanity as a gift of grace the lie which it had forfeited. This is the eternal testament of the Father, “the immutable will of God to give an inheritance to the believing”, and the eternal decree of the Father Himself, according to which He has promised the whole of humanity, so far as it accepts His grace with penitence and trust, righteousness and eternal life as an inalienable inheritance of grace.

Note the essentially universalistic basis upon which the idea of the covenant of grace rests. COCCEIUS introduces his exposition of the doctrine of the covenant of Grace (De foed. IV, 74), by declaring that of course God might at once have punished man with all evils. But the height of wisdom and power aided Him in His glorious plan for exercising mercy on man. Accordingly He resolved (1) to unfold His inexpressible mercy “in vessels of mercy”, and (2) “to employ an ineffable kindness and longsuffering towards the entire human race”.

–Similarly EGLIN (De foed. grat. 40): The “impelling cause of the covenant of grace” is the “love of God for the world, i.e. for the whole human race, which by the act of Satan had fallen into misery”. For this “common lapse” of the human race God had ordained in Christ a “common remedy.” Now a distinction must be drawn between “God’s general decree” and the “particular decree of election“. The former is (43) the “covenant of grace including the whole human race and by the counsel of His will planning for all indefinitely in Adam, on condition that man should repent and believe.” The latter on the contrary is the covenant according to which God “Himself graciously fulfills the condition required in those whom He has assigned to Christ from eternity”. As regards the former (44) the “promise is general by an outward calling“; as regards the latter we can only speak of an “effective application of the promise in accordance with the special promise of grace”.

Source: Heinrich Heppe, Reformed Dogmatics (Grand Rapids, MI: Baker Books, 1978), 371-372.

2) –OLEVIAN (30 f.): “The heavenly Father resolved so to execute the decree of His love as to satisfy perfect justice. As it is essential to Himself He can no more deny it than He can deny Himself. So in the actual execution of the righteousness the greatness and strength of His love in the Son and of His perpetual mercy sworn from the beginning had to shine fort”. Even in the Redeemer’s work of reconciliation its essentially universalistic side must be acknowledged. Hence EGLIN (54) declares that it must not be said that God did not send His Son into the world “to be the common Saviour of the whole world, conditionally set forth”. Only, the circumstance that not all attain to faith and blessedness must not give rise to the view, that “in proposing a common remedy” God had failed of His purpose (51): (i) “The things which. He decreed as ex hypothesi and conditionally must be estimated conditionally and (2) “in those whom (the Father) willed to give to Christ, fulfilled the requisite condition gratuitously”. But above all it must be insisted that it is not true to say that Christ accomplished nothing for the reprobate: “he broke and abolished the whole lapse of Adam, the whole curse of the whole law, in short, the one same enemy of all, Satan”. Undoubtedly therefore we may say (63) with Scripture, that “as regards all-suffciency of merit Christ made purchase even for the reprobate, and even for them paid the lutron of death in full considered by itself, although it is not applied to them by the saving attraction of faith”.

Heinrich Heppe, Reformed Dogmatics (Grand Rapids, MI: Baker Books, 1978), 373.

3) The single ground of the covenant of grace is God’s compassionate love for all men and the free Trinitarian counsel of God. –EGLIN (De foed. grat., 40): “The impulsive cause of the covenant of grace has been, God’s love for the world, that is, for the entire human race fallen into misery by Satan’s cunning”

Heinrich Heppe, Reformed Dogmatics (Grand Rapids, MI: Baker Books, 1978), 384. Full name: Raphaelis Eglini Iconi Tigurini (Zurich, Marburg) c1614.

[Cf., Zwingli and Bullinger, here.]