The connection of the Continental Reformed Churches to the Solemn League and Covenant (from an historical perspective) is simply that the SL&C was intended to bring these other Reformed Churches into union with the Reformed Churches of Scotland, England, and Ireland. How that would have been accomplished with different standards, I'm not sure. Would the Continental Churches have accepted the Westminster Standards? Or would they have retained their own standards? I have not read anything that specifically addresses that question. Perhaps the answer is that these Continental Reformed Churches would have participated in the drafting of the Westminster Standards and would have then acknowledged them to be their own standards as well. Nevertheless, it is certain that the Continental Churches (as represented by the Netherlands) were ready to unite with the Reformed Churches of Scotland, England, and Ireland in subscribing the SL&C. The SL&C was intended to bring all Reformed Churches under the same uniformity in doctrine, worship, government, and discipline.
As to the contemporary significance of these Continental Reformed Churches which are located in covenanted nations (e.g. Scotland, England, Ireland, Canada, the U.S. etc.), they should recognize their duty to subscribe the SL&C as individuals and churches that live within the covenanted boundaries of these nations. Had Continental Churches sought to establish Churches within the boundaries of Scotland, they would have been obligated to own the SL&C and the jurisdiction of the Church of Scotland. Those Reformed Churches (and Reformed Christians) that yet own the faith of their fathers should yet do what their forefathers were in the process of doing (i.e. owning these covenants and the faithfulness of the Westminster Standards).
One other historical note. A Dutch Reformed Classis ordained James Renwick (1683), a Scottish Covenanter, and even allowed him to retain his ministerial connection to the true Church of Scotland and to subscribe the standards of the Church of Scotland (the Westminster Standards, including the National Covenant and the SL&C) instead of the standards of the Dutch Reformed Churches. Such an act certainly indicates the unreserved approbation of the Continental Reformed Churches for the standards of the Church of Scotland.
Thus, it seems to me that the information concerning the covenanted reformation in Scotland, England, and Ireland is pertinent to all Reformed Churches regardless of their national boundaries, but particularly to those Reformed Churches that reside within the national boundaries of covenanted nations.
There was one great, and even sublime idea, brought somewhat indefinitely before the Westminster Assembly, which has not yet been realized, the idea of a Protestant union throughout Christendom, not merely for the purpose of counterbalancing Popery, but in order to purify, strengthen, and unite all true Christian churches, so that with combined energy and zeal they might go forth, in glad compliance with the Redeemer's commands, teaching all nations, and preaching the everlasting gospel to every creature under heaven. This truly magnificent, and also truly Christian idea, seems to have originated in the mind of that distinguished man, Alexander Henderson. It was suggested by him to the Scottish commissioners, and by them partially brought before the English Parliament, requesting them to direct the Assembly to write letters to the Protestant Churches in France, Holland, Switzerland, and other Reformed Churches. . . . and along with these letters were sent copies of the Solemn League and Covenant, a document which might itself form the basis of such a Protestant union.The deep thinking divines of the Netherlands apprehended the idea, and in their answer, not only expressed their approbation of the Covenant, but also desired to join in it with the British kingdoms. Nor did they content themselves with the mere expression of approval and willingness to join. A letter was soon afterwards sent to the Assembly from the Hague, written by Duraeus (the celebrated John Dury), offering to come to the Assembly, and containing a copy of a vow which he had prepared and tendered to the distinguished Oxenstiern, chancellor of Sweden, wherein he bound himself 'to prosecute a reconciliation between Protestants in point of religion'. . . . [O]n one occasion Henderson procured a passport to go to Holland, most probably for the purpose of prosecuting this grand idea. But the intrigues of politicians (like the covenant breaker Oliver Cromwell-ed.), the delays caused by the conduct of the Independents , and the narrow-minded Erastianism of the English Parliament, all conspired to prevent the Assembly from entering farther into that truly glorious Christian enterprise... (William Hetherington, History of the Westminster Assembly of Divines, [Edmonton, Alberta: Still Waters Revival Books], pp. 337-339, book at http://www.swrb.com/catalog/h.htm).