Heh. That sounds like the premise of the Avengers: Earth's Mightiest Heroes cartoon (the prison break of supervillains, that is). It worked there, so I don't see why it wouldn't fly for a game.

As far as lobsters go... might I suggest coconut crabs instead? They're already huge, so it wouldn't take much to make them man-sized, and they've got a visceral terror factor going for them that I don't think lobsters can match (for example, one theory on Amelia Earhart's total disappearance was that after she crashed, her body was torn apart and eaten by these things). They can also carry 7.5 times their own body weight, easily exert enough force with their pincers to crush a human skull, and climb trees. And that's at their normal size- bigger ones would just be straight-up nasty.

I'm not sure if it's too much to put on the players or not; you know your group and I don't, there's not a lot I can say there. The usual 'have a session zero and get buy-in from your players' applies, but beyond that, I don't know, sorry. Although I will say that if you like a concept, go for it. Don't necessarily burn a lot of time on it or anything, at least at first, but don't discard an idea just because you're worried things won't work out.

One other bit of advice I'd offer, just on the very vague impression I've gotten of your group... I would suggest the main 'punishment' for non-heroic behaviour be the loss of the ongoing rewards they already get for being heroic, rather than any specific bad stuff happening to them. Although I would make certain that they actually want to be heroes for this game- if it's not a premise they're interested in, no amount of rewards or punishments will drive them towards it.