One random question: Would it make sense to power the system from an RC NiMH battery? I found one that outputs 1700mAh of 4.8V for the same price or less as a bunch of eneloops to do the same thing with less mAhs here.
How do you charge these batteries, though?
BTW, I don't think I'll want an in-bike charging circuit, as I have read that Nickel batteries should only be charged at room temperature, and, well, winter's coming. I am thinking just to be able to alternate between dynamo and battery mode, without ever connecting the battery to the dynamo itself, instead taking it to my room to recharge.
Annoyingly, the computer I am on will not let me view that link so I'll have to look at it later... Most RC NiMH batteries can be bought with a charger and that would probably be the easiest option. If you can get one that it over 5v then it would be better (6v ish). As the voltage of that battery is less than 5v, you are going to need a DC step up circuit still...
Charging NiMH batteries:
Mowcius