Kremmit - the main problem I see is that when you move the joystick, you are probably going to bind up those gears, I think, maybe not. I can probably be done, but not easily.
Why would they bind up? They're just floating in space, after all!

Seriously, that pic isn't meant to be
the actual mechanism, just a visual explanation of the concept.
To actually build it, you'd need the switch and it's gear to attach to a bracket that is in turn attached to the shaft itself. That way, the gears will move with the shaft and not bind up. Same bracket should also hold the leaf (or micro) switch on the bottom for the top-fire. And of course, then you'll need
that bracket hooked into an anti-spin rig similar to the ones in fl0yd's rotary kit.
I can
see this magic bracket in my head, but it's more then I felt I could tackle in Photoshop.