The latest seller update did say that buyers who lose a case cannot leave feedback.
I spoke to the resolution center and was told that the buyer needs to call them, since it is him they would be helping, but I would automatically win the case if he escalates it because I fulfilled my obligations.
Of course, unless ebay buyer protection pays out in these types of cases, once he is told we would lose if he escalates, why would he bother? So he wont escalate it, which means he wont lose, which means he can leave whatever feedback he wants!
I'm hoping that it's one of those situations where ebay pays out to the buyer even when they are siding with the seller - I believe I've read of cases like that in this thread.