I've got a Monty X-hydra that's really good at showing me how bad I am.
15 minutes on it can be a heck of a workout.
You can ride it around a basement and have a blast.
Geode: the geometry is pretty different from a BMX, plus the gearing is really low on the trials bike, plus mine has a bash plate.
That said, you can work on some basics on just about anything.
Most of the guys I've hung out with ride urban trials on homebuilt 26" bikes that have a jumping pedigree.