Andy, I would be interested to hear how you get on with this one because I thought about doing the same thing before, but never started it.

Its possible that you could also incorporate ABS into the same unit by applying reverse motor current when a braking signal is received, and pulse the current if the wheels are seen to lock by the photodiodes.

I had exactly the same problem with voltage sag on an ESC that I build for a twin motor monster truck (TXT-1). What I did was to go out and buy one of those 1 Farad memory retention capacitors and have this charged by the Nicad battery. It supplied power to the PIC only (not servo's) by way of diodes etc. The thing to watch is that your PIC doesn't have to source much current to other things, e.g I was using the hardware PWM module to control motor speed.

Let us know how you get on.

Andy P.