Without going into the details, you could hypothesize a simple mind than automatically rejects any argument. This would by itself prove the No Universally Compelling Arguments theory.
That would do it, though it may only attack a straw man: the thesis that the, say, categorical imperative is universally compelling is not the thesis that the CI is universally persuasive. Rather, I think the thought is that we are all rationally committed to the CI, whether we know or admit this or not.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.