That would be really cool... although I doubt it would be very affordable. Sorry I don't have any clue who you can partner up with for such a project. I would imagine it would have to be a little bit of everyone up and down the supply chain. Assuming you are thinking about something more involved than a replaceable battery stash on the side.
I was thinking the other day as I was looking at my battery usage stats:
>Why does the CPU have to run at full-tilt processing speed all the time? Can the user slow it down to conserve battery? or can it be made to run more slowly for basic apps and functions that do not require it?
>Why does screen resolution and color graphics have to be pegged at max all the time? Why can't the user dial down both to conserve battery?