Sorry about the delay responding. I ended up buying one of these:

http://www.bike24.com/p238306.html

It has a small on-board battery that's used to regulate the voltage. It's a bit more inflexible in that it will only charge USB gadgets but it should suit my needs. Can always use it to charge one of these:

http://www.amazon.com/Thunderbolt-In...battery+backup

if I want to charge non-USB gadgets. Still hasn't arrived and will then have to get the wheel built. I'll post how it goes.