I think you will still need the USB power hack, sadly.
I have tried using:
Digital camera
Nokia USB phone
Powered USB hub
Powered USB CD-ROM drive

And none of them were recognized as USB devices. Yes, I had the 770 in host mode. In most cases, it appears that while there may be an external power supply for most of the system, the USB interface chip is still powered by the USB port.

You may get lucky and find a device that works, but chances are you're still going to need a cable and/or internal power hack.