Hey freak, I checked out your build thread.
Instead of a detent ball lock system, how about having the jockey shift lever run along a guide plate that had cut outs shaped kinda like a lightning bolt to catch and lock the lever in any position you want.
The jockey shift levers hinge pin would just need to have a little slop (so the lever would tilt L/R a bit for shifting) and then have a strong spring pulling the jockey lever in towards the lightning bolt shaped detents in the guide plate.
So you'd just grab the jockey, move it sideways a bit to unlock, make your shift and it would automatically lock back into place wherever you had a notch cut into the guide plate.
I'd cut a bunch of extra notches into the guide plate to compensate for belt wear...
(edit: whoops, i see R97 already suggested the same thing while I was searching for my lightning bolt pic