While I'm sure a custom mouthpiece isn't ideal, I'd bet the shape/comfort of the mouthpiece would be pretty low on the list of priorities for someone who is out of gas.
I've not heard of a regulator being unusable in an airshare because of a seacure.
This came up in the DIR forum a while back (or at least the person in question was DIR). One poster (poser?) said that he'd probably choke and drown if he had to use somebody's custom seacure mouthpiece because his mouth was so small and stated that it wasn't safe for "the team" to dive with one.
Do a search, it'll probably come up.