First off, it seems to me that to make any sense at all of disgust playing a justificatory role in moral epistemology we have to think of it (and other cognate attitudes, emotions, etc.) as being a cognitive state, some sort of judgment or belief-like state with specific intentional content. Otherwise, it's hard to see how the emotions can be truth-apt, as Crispin Wright might say.
Perhaps I am just overly Humean, but this strikes me as utterly false. It is easy to find cases in which a justificatory role is played by something that's not a cognitive state. Necessity is the most obvious example. To borrow from Hume, if we recognize nature has determined us to judge as well as to breathe, it becomes very silly to ask in reply, "But what is our justification for judging?" If you can't not judge, any sense in which you need a further justification for doing it is just a bizarre sense, one we have no reason to take seriously. Disgust doesn't straightforwardly necessitate in this way. However, it is a fact of human nature that any moral epistemology has to take into account: human disgust affects moral reasoning. And what is more, it actually does play a justificatory role in human reasoning; people really do appeal to it to justify reasoning in particular ways. But even more than this, it appears that it is virtually inevitable that it do so (the incest example is a case in point). The only question, then, is not, "Does disgust play a justificatory role in our moral reasoning?" but "What justificatory role does it play, and what are the limits of this sort of justification?"
I also find the idea that truth-aptness requires a cognitive state to be rather strange in this context. The only sense of 'truth-apt' that I can see to be relevant for justification is 'makes it more likely for us to make true judgments'. There is nothing that is intrinsically truth-apt in this sense: even logical inference is not intrinsically truth-apt. Truth-aptness arises under particular conditions; and there appears to be no a priori reason why sentiments and passions could not, under certain conditions, make us more likely to make true judgments. The only question is: Under what conditions are they truth-apt? It can easily be seen that this is the same question asked above, in different words.
