You could probably use this instead of a Camelbak:
http://www.voltaicsystems.com/bag_converter.shtml
Granted it won't be a comfortable on your back, or shed heat as well, but...
I have the large 15W version that I set on top of panniers and I can string it up to run my GPS fulltime without needing to drain the GPS batteries (it would otherwise eat them up about every other day). It's expensive, but the nice thing is it will store excess charge in the battery which I can then use to charge a number of different devices with the standard connector set. They have additonal connectors if you have something that the standard set won't charge. I've somewhat abused my bag, and the company has been very accomodating to fix me up with a replacement.
FYI, with the bag I have I can actually run a 17" inch laptop for about an hour off the included battery.