I see. There are a few options.
First, probably simplest, is to just create a few boxes. A thousand of static boxes or simple hulls can probably be mangled at reasonable speed. My current world is about 400 boxes and 200 hulls, ticks at about 100 fps on my system (Athlon X3 450).
The next step would be to collapse nearby boxes in a single one. This will also improve stability.
Note, as showed here
bullet silently assumes rigid bodies do not touch each other. When this does not hold true, there will be issues with simulation stability (although predictability is probably a better term).
To solve this, I have played a bit with polygon soups (they allow internal edges to be properly detected)... I couldn't quite make them work, in general, I think the best thing to do is to have a proper kinematic controller taking care of those cases. As for dynamics... well, I guess that's a problem.