All that is needed is a Y cable that has the following connectors:
1. 5-pin male (type B?) to connect to the 770
2. Female type A to connect your devices to
3. 5V input

1. and 2. should have all four lines + shield connected; and connect 3. to their +5V and ground lines. Add a 9V->5V or a 3V->5V convertor, the appropriate batteries, and you have portable usb host access. Very handy indeed.