I was wondering about using this;
4X AAA capacity, battery power indicator lights, comes in several colors, and you could just switch out the led for a connector. They also make some that have a usb charging plug on them to charge the nimhs with, but those look a little more complicated. Do a search for NDS or PSP emergency charger for additional units.
Nice find! The only boxes I've seen like this are so you can use NiMH batteries to charge things like an ipod or cell phone. The USB is output only and the batteries have to be removed for charging. NiMH's want to be charged individually, not in series because they don't charge equally.
It will still make a great looking mod... I wish I could charge the batteries w/o removing them but it's difficult with NiMH's.