You need to build a power injector. While you can put the USB port on the 770 into host mode, it won't supply the 5 volts required.

Google "thoughtfix 770 USB hub" and you should find a link to Thoughtfix's plans for one. While it's dealing with electronic components, it's very easy and cheap to put together. Note that voltage regulators still draw power even if the device isn't on so you may want to put a switch in there. (My 9 volt died in a few days because of this).

*** note: I should mention that my 40GB mp3 player acts as a flash drive when plugged into a PC. I was able to mount it and play movies/mp3s off of it no problem.