[Otaku]

I tried hacking a BB a few years ago to make a bush shaker. As you found, the motor and battery pack are the counterweight spinning around the central shaft. The BB switch isn't the usual affair, either. The only way I could do it reliably would have been to buy an expensive rotating mercury coupler and insert that into the shaft, taking the electrical contacts from the coupler. Any other way of wiring a remote trigger to the switch would result in the wires getting wrapped around the shaft. I did have some success using alum. foil as a contact material, but it wore out pretty quickly. I gave it up and used a large vibrating motor for the shaker.