Lessons learned

Geoff and I got the back up system working. As Simon predicted it is simple and it worked. Geoff's pedal arrangement can drive one wheel or all three together.

However! We learned a few things:
1) Fuses are good! We discovered this burning through several 10amp fuses until we realized that the diodes on the contacters were causing a short. The diodes are in the right direction for the main control, and the wrong direction for backup mode.
2) Disconnecting the power completely before messing with wires is highly recommended. We burnt through a thin wire that accidentally shorted with a loud ZZZZAAAPPPPPP which scared the hell out of us.
3) 24V will probably be too fast. But we can re-wire it for 12V very easily.
4) Chains are loud. Sorry. I shoulda used belts or gears.
5) A kill switch that cuts all power at the main fuse panel is pretty essential for safety, i.e. the backup control system has no "Off" switch without it.