sorry to bump!

This is a really elegant design, and I recently built a similar one (though, without a switch - alligator clips to connect also serve to turn it "off" when I want, and with my lights outside of the DC, running off the hub in parallel. 1/4" fastons with rubber dust covers provide the clipping point. A velcro strap integrated into the charger's housing holds the thing to the fork blades to keep it out of the way. Other notables include a fancy positive-locking usb port, because that sounded like fun, as well as a rubber dust-plug to keep it safe when it's empty. Next time around, I think I'll use a battery holder sized for AA batteries with flip-down tabs to accommodate AAAs - versatility to the max, but I couldn't find one when I was working on this.

I'd also recommend soldering D+ and D- together in order to let the device know that it's just a charger, not a computer.

But my main use case is charging things after I'm done riding - removing the battery pack and powering a device when I leave the bike behind. The thought occurred to me today that there's a potentially dangerous situation - what happens when the AAA batteries are drained? Will they suck the device's batteries dead? Should the circuit include, perhaps, a Zener diode between the AAA and the USB port? I suppose this is the device's responsibility, not the charger's, but.....
thoughts?

Also interesting: when I have both my light and the charger running at the same time, the light is dimmed considerably, maybe to about half the appropriate power, just a point of interest, not a big issue. Son 20 in a 700c wheel with (first generation) E3 lamp and tail light.