[Al Skierkiewicz]

Matt,
You are correct, the power dumped by the backdriven motor into a speed controller is lost as heat. Part in the speed controller and the rest in the wire (both motor internal and external) The speed controller is capable of 600 amps so I don't think you will ever get to the point where the speed contoller is at failure. The current generated by the motor is a function of how fast and how much wire is passing through the magnetic field inside the motor. The current is higher at high RPM. While the current is climbing, the back EMF and the braking effect are also climbing. There are so many variables involved, you may not be able to accurately predict the results. If you wanted to build power resistors into a custom circuit board box with some relays, you may be able to get variable control of braking. I am just thinking off the top of my head here, but it might be possible.
Using braking on some motors in a multimotor drive is a tricky thing to do, if something goes awry you are producing your own load and the current skyrockets in the driven motors, resulting in tripped breakers.
Good Luck