yes, the 28V is required for the logicboard to work. (i tried a DA and a QS with 12V instead of 28V - didnt work). maybe there is a hack, but i never found out.
as far as i know, this is valid for all G4 except the sawtooth.
fyi: the G5 models seem to boot nicely with a converted ATX-Powersupply.

shame i can't find that page with all the pinouts again. every model has its own pinout, so watch out.