I know a few have wanted this and never saw a solution posted. I did a very small write up on hbf:
http://www.hpibajaforum.com/forum/sh...=1#post1433342
All it is, is using a picoswitch(or equivalent) on ch3 to turn on and off the power to your brake servo. Shut it off to back up, and back on to run.
Did I do good?