Well, he's still alive this morning. I am misting him frequently, plus the container is very moist inside. Somebody left the kitchen light on all night, so both snails were "asleep" when I got up this morning. I stuck the container in a dim corner of the living room, and now the damaged one is moving around a little.
This is like a little science project. Can a snail regrow so much of its shell? How long will it take? And I think his saving grace was that the whorl-part is still intact and attached, but I'm still not expecting him to survive.