Really?!!!! Name all the "philosophical naturalists" you know of who have espoused the idea of eternal laws of nature.
I definitely find it difficult to comprehend in what sense the laws of nature, e.g., Coulomb's electrostatic force law, supposedly existed 100 trillion years ago, before there was a universe. Obviously the proposition that the laws of nature are eternal is not deduced from any fact. In what way is it different to claim that the laws of nature are eternal and to claim that they exist(ed) in God's mind?
The SEP article on Laws of Nature does not mention anyone espousing the eternalness of the laws of nature.
The IEP article uses the adjective once in passing, without citation. This article is written by Norman Swartz. Anyone familiar with his thesis that rests on the distinction between regularist and necessitarian views on the laws of nature will readily understand his emphasis of such distinction in this article. He verges on caricaturing the necessitarian position in this article, and overtly rejects it:
Even as recently as the Eighteenth Century, we find philosophers (e.g. Montesquieu) explicitly attributing the order in nature to the hand of God, more specifically to His having imposed physical laws on nature in much the same way as He imposed moral laws on human beings. There was one essential difference, however. Human beings -- it was alleged -- are "free" to break (act contrary to) God's moral laws; but neither human beings nor the other parts of creation are free to break God's physical laws.
In the Twentieth Century virtually all scientists and philosophers have abandoned theistic elements in their accounts of the Laws of Nature. But to a very great extent -- so say the Regularists -- the Necessitarians have merely replaced God with Physical Necessity. The Necessitarians' nontheistic view of Laws of Nature surreptitiously preserves the older prescriptivist view of Laws of Nature, namely, as dictates or edicts to the natural universe,edicts which -- unlike moral laws or legislated ones -- no one, and no thing, has the ability to violate.
Regularists reject this view of the world. Regularists eschew a view of Laws of Nature which would make of them inviolable edicts imposed on the universe. Such a view, Regularists claim, is simply a holdover from a theistic view. It is time, they insist, to adopt a thoroughly naturalistic philosophy of science, one which is not only purged of the hand of God, but is also purged of its unempirical latter-day surrogate, namely, nomological necessity. The difference is, perhaps, highlighted most strongly in Necessitarians saying that the Laws of Nature govern the world; while Regularists insist that Laws of Nature do no more or less than correctly describe the world.
In any case, regardless of the fact that you are saying something directly contradicted by the scholarly literature on the matter, to claim that the laws of nature are eternal certainly doesn't refute the cosmological argument in any way. Indeed, it would seem to support the cosmological argument.