So, the Bayesian translation of his position would seem to be that he has a high prior on zombies being conceivable. But of course, that in turn translates to "zombies are conceivable for reasons I'm not being explicit about". Which is, naturally, the point: I'd like to know what he thinks he knows that I don't.

Regarding coherence, and reasons to believe it's false: the historical success of reductionism is a very good reason to believe it's false, it seems to me. Despite Richard's protestations, it really does appear to me that this is a case of undue reluctance on the part of philosophers to update their intuitions, or at least to let them be outweighed by something else.