I would think they could hold some pressure- that is, after all, how it was done "back in the day." But I would think there's a reason why the cask ale genre is served at a lighter carbonation. I'd try it, the only thing you could lose is some beer or at worse a barrel. I'd be surprised if it could hold enough pressure to become dangerous.
It'd be cool to try the beers side-by-side.
"If you’re stressing over homebrewing, you’re doing something wrong." -- Denny Conn