I don't think a "dummy battery" would do the trick, as the N800 seems to be able to draw more power than the wall wart will provide. I know about a week ago I was playing an internet radio stream, and despite the fact that it was plugged in, the power eventually ran out. At that point, it rebooted into the "charging" screen and proceeded to start recharging.

Though, more tests could certainly be done to figure out where the break-even point is.