You could just use capacitors to step any voltage up .
2 batteries running in series ~2.8 volts constant with 2500 mah
2 batteries running in series stepped up to ~5.6 volts with 1250 mah
Depending on how many amps you can pull off a AA without blowing it up you could pull the power from them for anything like a usb based wifi adapter or a mini hard drive .
Granted the runtime would be affected but this really is the best solution .
I would say the ultimate goal of this is not to only have portable powered usb but to also have an intergrated WiFi . That would change the entire concept behind the gp2x . Could you imagine streaming movies ? Or internet tunneling engines running completely off a gp2x ?
I'd really like one of these, but I'd also like a USB host cable with an inline 3V to 5V stepup converter so you could use low-amperage devices like the DreamGear or iType2 USB thumboards or joysticks without any additional hardware. I was thinking of getting one of these to hack something together: