Oh, regarding the lights themselves, visit REI if possible, or any store that lets you handle the lights. That way you can see which have readily accessible USB ports. (And buy from that store if their prices are reasonable -- I bought my lights from REI after playing with their toys because they have a great display board for customers.)

The Cygolite Hotshot USB ports are easy to reach, on the bottom of the lights. The Hotshot 50 uses a supplied mini-USB port. Some of the others use the smaller micro-USB port. For awhile I had my Hotshot mounted so that it couldn't be easily detached -- it was nearly butted up against the bottom of the saddle so releasing the quick release clip still wouldn't clear the saddle. I'd just recharge the light on the bike.

The Light & Motion Urbans have a USB micro port under the front of the barrel. Fairly easy to reach without detaching the light.

I also use, and like, my Blackburn 2'Fer light on the back of my helmet, but it must be detached to access the USB port. The Vivo Bike Illuminati I use as a front helmet light can be accessed without detaching it -- much handier. But it's not really bright enough to be a sole headlight.