Fascinating thought Tom. Could it be a simple rigging some thing like this between the dyno and battery?
6 Volt 4 Ah Power Wheels Battery Charger: BatteryMart.com
if that would work, yeeha! Light and small.
Is to be used with the soon to be patented "2 Door motor bike lighting system"
(Tom made a headlamp from an emergency hallway light. The ones you see in office and apartment buildings that come on when the power goes out. Is really kool and was waiting to do a thread on it until I actually put it on a MB)