[RC-Monster Mike]

The concept I envision would be a hybrid setup - mechanical rear brakes only, with a mini or micro servo, mixed into the 3rd channel. You could use a smaller servo by only using the mechanical rear brakes, and you could still effectively adjust front/rear bias completely with your radio. The motor and speed controller would brake the entire diff, and the rear mech. would obviously brake the rear. If you needed more rear bias, you could dial(mix) in more rear brakes on the 3rd channel, or dial out brakes on the 2nd channel(controller brakes).
This setup would save some weight, add some space, allow electronic bias tuning without leaving the driver's stand, and take significant load off the speed controller and motor during braking. I think this would be the best compromise between esc braking and mechanical braking - lighter than pure mechanical brakes and easier to tune, and more tunable than esc braking alone. :)