It just occurred to me, even if you ground out the antenna, you'll still have the problem of locking it in forward. I'm not sure how you overcome that one.
This is a long shot, but if you used a DPDT switch and a rectifier you might be able to bypass the electronics altogether and hardwire the motors in forward. The other side of the switch would feed power to the electronics for TMCC mode. You might need some diodes on the motor leads from the board to prevent feedback into it.
It begs the question, how badly do you want this?
