Yes, it's possible and I've done it. But it's not easy.

Basically the USB port on the 770 doesn't have 5v running through it like the one on your PC does. You need to build ($20 or less) a USB power injector (search for 'USB power', thoughtfix has an informative thread on this).

I was able to mount my 40GB mp3 player. Note though, the 7805 voltage regulator uses up power even if off, so make sure you put a switch somewhere on there to turn it off when not in use.