Constraints and soft bodies

Hello, I was wondering if anyone using sio2 has come across using constraints on soft bodies?

I see in blender that you can add "rigid body" constraints to soft bodies and they work in the game engine, but bullet does not seem to have a way to add constraints to a btSoftBody.

Looking at the bullet btSoftBodyHelpers, it seems like they set the mass of the constrained vertices to 0. I did this in sio2, and those vertices remain stationary now, but the rest of the vertices continue to fall and the edges between the stationary vertices and the regular ones just keep on stretching. Anyone have any ideas?