I have thought for ages of something like this, but instead of usb drivers, a wifi box with a big harddisk. You would connect to the wireless network, open browser at 192.168.0.1 and instead of a router's page, you would get to a website, with a nice interface, where you could select the files you wanted to download or upload others.

I even did some drawing and thought of solar pannels and batteries to keep it going non-stop. What demotivated me was the fact I'm living in Brazil and normal filesharing is still much easier and non-risky...

Anyway, a wifi box like this would be much more secure if done right. A complementary idea I also thought of was to make the boxes communicate with each other, much like a cloud you could access from any reachable access point.