Since it's not in the Bible, are Evangelicals taught to ignore or disapprove of the teachings of such folks as Aristotle and Plato? Would they belive a redacted version of the Summa Theologica or City of God where all the Aristotelean and Neo Platonic stuff is gone and only the Divine Law stuff remains?
One of the founding ideas of Protestantism was "sola scriptura", meaning that we view the Bible as the only ultimate authority. That doesn't mean that we refuse to read any other books. The existence of thousands of Baptist bookstores should be adequate proof that that's not true. What it means is that we do not view any book other than the Bible as having any authority beyond the weight of the writer can convince us of by the force of his arguments.
Frankly, I suspect most rank-and-file Protestants don't know who Aquinas, Augustine, etc are. (Well, does the average Catholic know who they are?) Among the more intellectual Protestants, of course we read and quote and discuss Greek and Scholastic philosophers and the Church Fathers. We just read them with the attitude that we will agree with some things they say and disagree with others. We don't need "redacted" versions -- we're grown-ups, we can handle the fact that we may not agree with everything we read. :-)
"The idea of 'Natural Moral Law', if you are referring to the idea that one can discern moral truths from pure philosophical pursuits is something I would argue is untenable."
I would suggest that you read Cicero's "De Legibus" ("On the Laws"). Within, you will find that Cicero arrives at many conclusions about of God and His law which approximate Scriptural truth. They are limited and subtly different and for this reason, they are more dangerous at turning a Christian aside than blatant contradictions. It is for this reason that the early Church was turned aside to various forms of "Hellenist" Christianity. Nevertheless, your statement would be found by Cicero and others as untenable. And the conclusions of these pagan thinkers demonstrate the verity of the 1st Chapter of Romans (v 19-20) that an unbeliever can know some things about God from natural revelation, however difficult and unlikely that is. This is important because it means that there is often a point of contact in proselytizing. We don't have to rely on signs and wonders, presuppositionalism, gnostic and pietist witness and Pascal's Wager.
I am not sure about the "Evangelical take", but as a disciple of Christ who studies and lives and speaks in communion with other Christians, I have come to certain conclusions about truth.
The idea of "Natural Moral Law", if you are referring to the idea that one can discern moral truths from pure philosophical pursuits is something I would argue is untenable.
Scripture (the most extensive revelation from God) is the foundation of any attempt to find truth. Reason is a way of interacting in our limited and flawed capacities with ourselves and others to test and affirm the validity of our understanding of Scripture and explore it and comprehend it, though ultimately imperfectly. Tradition passed down orally and perhaps even by texts and creeds, etc. in an unbroken line from the original eyewitnesses to the history of God's interaction and revelations with people is also paramount. Finally, the indwelling within our earthly vessels of the Holy Spirit provides direct guidance from God for each of us in our search for the truth, in concert with all these others things.
Spiritual truth ultimately can only come from God, including moral truth. Philosophy when reduced to pure mental exercise is fundamentally arbitrary mind play, and can never lead to truth if used alone. No moral stance can be extracted that can be considered truth from it without proper spiritual guidance. One only need look at Nazi Germany to realize that they had a very elaborate moral philosophy that, though thoroughly reasoned through and applied by some with real belief that their way was the best way for all on earth (though of course especially for Germans) to understand the horrible outcomes of reasoned morality through philosophical endeavors not predicated on revelation from the one true loving God.
One also can cite the proliferation of endlessly competing and disparate though thoroughly well reasoned world views espoused throughout the ages to realize that without guidance from God's authoritative truth we are as a race helpless to define meaningful moral or spiritual truth.
When tempered by the four pillars used to explore God's truth mentioned above, however, and guided by the Holy Spirit, reason and philosophy that comes from it can be very valuable means of broadening and deepening ones understanding of the one true truth as revealed by God.
By sharpening the mind and exposing those who spend their time in theological pursuits to alternative though possibly flawed points of view, it can be valuable as an exploratory mechanism to give counterpoint to the truth that might lead to insights for an individual or in corporate study of elements of the truth not realized without some critical thinking. It can also become just a waste of time, though, if done too much without a good dollop of Scripture, tradition and the Holy Spirit!
Unless we have an understanding of philosophical methods, we would have no idea of the validity of any work claiming to be an exposition of the Bible.
One could read the Summa, but unless you know how it lacks, it would be pointless. The Divine Law, its requirements, the right approach meet those requirements, all this require a proper understanding of the problem, an area that Hellenistic philosophy fails in.
The Scholastics utilised reasoned, logical thought in their methodology, arguably the basis for modern biblical hermeneutics, the same used by Luther, Calvin and Jonathan Edwards.
Yes, read the Summa to see how the arguments are made, but be aware of the Greek Anti-material Dualistic basis of those arguments.