I never saw this before, but you could actually build most of nonfoundationalist ethics on proposition 6.421.6.4 All propositions are of equal value.
6.41 The sense of the world must lie outside the world. In the world everything is as it is, and everything happens as it does happen: in it no value exists—and if it did exist, it would have no value. If there is any value that does have value, it must lie outside the whole sphere of what happens and is the case. For all that happens and is the case is accidental. What makes it non-accidental cannot lie within the world, since if it did it would itself be accidental. It must lie outside the world.
6.42 So too it is impossible for there to be propositions of ethics. Propositions can express nothing that is higher.
6.421 It is clear that ethics cannot be put into words. Ethics is transcendental. (Ethics and aesthetics are one and the same.)
-- Ludwig Wittgenstein, Tractatus Logico-Philosophicus
If I understand Wittgenstein correctly here, he's saying that ethics in order to have meaning or truth-value, they must be transcendental - that is to say outside the world and beyond language.
This is also the first principle of nonfoundationalist ethics. Starting from the idea that God either does not exist or does not reveal Himself in the world to stand as guarantor for a transcendental code of ethics ("6.432 How things are in the world is a matter of complete indifference for what is higher. God does not reveal himself in the world."), it follows that there is no higher standard to which to hold any action. It follows from this that the rhetorical actions of the public sphere are where all ethics take place. It's all just us judging ourselves, and that's where we need to take our opinions about what should be the cultural standards.
I quoted Wittgenstein in my high-school yearbook. God God, I was pretentious. Glad I'm totally over that phase now.6.52 We feel that even when all possible scientific questions have been answered, the problems of life remain completely untouched. Of course there are then no questions left, and this itself is the answer.
6.521 The solution of the problem of life is seen in the vanishing of the problem. (Is not this the reason why those who have found after a long period of doubt that the sense of life became clear to them have then been unable to say what constituted that sense?)
6.522 There are, indeed, things that cannot be put into words. They make themselves manifest. They are what is mystical.
6.53 The correct method in philosophy would really be the following: to say nothing except what can be said, i.e. propositions of natural science—i.e. something that has nothing to do with philosophy—and then, whenever someone else wanted to say something metaphysical, to demonstrate to him that he had failed to give a meaning to certain signs in his propositions. Although it would not be satisfying to the other person—he would not have the feeling that we were teaching him philosophy—this method would be the only strictly correct one.
6.54 My propositions are elucidatory in this way: he who understands me finally recognizes them as senseless, when he has climbed out through them, on them, over them. (He must so to speak throw away the ladder, after he has climbed up on it.)