Flingwing207,
If you lower the collective fast enough, you can reach zero g, and it might be possible to create a problem even if you do not make much pitch down, too, but it would take one hell of a down collective motion.
I don't want to rule out a "collective only" zero g event, although it is far less likely than an event that includes significant nose down motion, as well. Remember, if you down collective and do not use enough aft cyclic, you get both, which is the place you want to avoid.
I also think that the roll contributes to the mast bumping problem, but once at zero g, the mast bumping can occur for other reasons. Significant sideslip (badly coordinated pedal) can also make the flapping go toward limits in the 204, 205 and 206, so I imagine it can in a Robbie as well. Thus, extreme dumping of the collective might get you to a mast bumping, as well, especially since it involves a chance to mess up the pedals while getting to zero g.