You might consider a locking differential/torque sensing differential to help balance the braking force.
The reason most cars did not use this type of brake is that the drive line turns 3x(Depending on the differential ratio) the speed of the wheel so stopping from 30 is the equivalent of stopping from 90. Generates significant heat. Use a Carbotec or similar pad from a race car and you should do better, also a duct to provide air to the rear brake is likely a must have. You'll also be getting major heat pumped into the pinion bearing from the rotor. A finned rear diff cover, with the duct blowing air across it will help. There is paint you can buy that changes color at certain temperatures, Might be a good tool for checking the function of the brake and caliper in use without a lot of high tech instrumentation.
I'd put the electric motor between the trans and drive line, Replace the tail shaft housing with the motor.. Only because its way too easy to get harmonics on the crank when you hang stuff off it. It also lowers the effective rotational leverage of the unit on the engine under acceleration, race shop I work at we put alternators on the drive line on some cars for that reason. Raises straight line speed significantly ,, more speed less load...
Dave