I've added the full-order Blackbird model to the PyBullet simulation. 

It's running the same SLIP controller as before, but the Blackbird's torso and feet are constrained to the positions of the SLIP walker's torso and feet. 

This means the assumptions of the SLIP model are no longer true (the legs are no longer massless), yet it performs admirably with no modifications. I believe this proves something about the inherent stability of my robot/controller design.