For my part, I am a bit skeptical about McSoreley's absolution of the young Thomas on this point. My reading of Thomas (as well as others) would suggest that he became more Augustinian as he grew older and that his immature theology is in fact Semi-Pelagian. Some scholars even claim that he discovered a copy of the Council of Orange I and II mid-career, and therefore he changed his mind. I don't think this is true either, I think he just read more Augustine's anti-Pelagian writings.
In any case, the principle of Facre quod in se est would indeed make sense as being a common Scholastic principle, even if one posited that it occurred via the assistance of divine grace. In other words, if one accepts the premise of Aristotelian metaphysics that matter must be disposed to the form (in this case, the habits of created grace), then one would have to posit some sort of preparation for justification. Hence, although Trent rejects the idea that Facere quod in se est works on the basis of humanity's natural powers alone, it never the less insists on a lengthy and extremely complicated series of stages wherein one is prepared for justification by divine grace through "prevenient," "healing," and "elevating" grace.