Meh! I had a "mind the gap" solution that
* was gravity-powered (of course in this thread),
* had the ball unattached,
* put it at rest in the pink zone,
* and (for extra sport) left it alone there.
It broke since yesterday. Must have been an update. And since the last point took an insane amount of tweaking (and, considering how much it fails in the newest version, how difficult it would be to patch it up), I won't be updating it...
Anyway, here is the idea of the design.http://fantasticcontraption.com/?designId=43606