Yep, already thought of that. My ideal would be to hack off the USB keyboard cable and put a mini-USB-ended cable on with the appropriate batteries connected at the keyboard. Probably need to add a switch for that matter to avoid draining the batteries the entire time. That way there would only be a single cable to plug directly into the 770. I was at Walmart today and they sell a "PSP USB" cable that is really just a mini-USB->USB male cable like the Nokia one. The cool thing it's only $5, so you can afford to hack away with having to buy expensive cables.

Incidentally I'm just finishing up the initial mke2fs of the 30G USB hard drive I have hooked up. I purchased a USB enclosure for a laptop drive at Fry's ($20) and thought it would really be cool to have a "man-sized" bit of storage for the 770 :-)