You could mix the throttle channel to ch3, and have the mix on a switch. This is similar what Brain said, but I believe he implied just using a button for ch3 without a mix. Continuing on though if you had the switch normally on (mix on) then it would be the same as driving with brake, and no reverse. However if you find yourself stuck and need reverse simply flick the switch (turn off the mix) brake servo returns to neutral and your right to reverse. No different to changing your car into reverse. Simply flick the switch again when your going forward and manual brakes are back on for when you need them. You would have to remember not to move the trigger back into reverse without turning off the mix though as you would get the motor fighting the brakes. I sure with a fancy enough radio though you could set it so it alerts you or automatically disables the mix?

Kieren