Even if the Nokia 770 generates +5volts for it's own circuitry, it would be a bad idea to try to tap that power for external USB devices.

There are a couple reasons besides battery drain:
-External USB devices could pull much more power then the 770 could provide (not due the battery life, due to the internal power regulators output)
-There no protection on the USB jack if a device were to pull too much power, or short out. (Normal USB jacks on PC's have this protection)
-Most of the internal circuits on the 770 probably run off 3.3volts not 5volts.
This allows them to work most efficiently off a lithium battery.

So you will still need to do the external power injection modifications on the 770 in host mode, as has been posted in the forums before.