Well, i don't think this is a bad idea, but the project shoud be able to handle PCIe video cards, PLUS not have the CPU embedded, being able to be replaced for a more powerful one by the user. To be able to fit a 2ghz dual G4 would provide a more than enough power for all our needs... combined with an efficient video card of course. Specially since G4 Mac Mini, has poor video capabilities and and sort of a short CPU computing power.
However i don't know if after MOS is ported to the macmini G4, the team will head over the rest of the Apple's G4 architectures... If so, i wonder if a new board having same capabilities than these discontinued systems could compete under pricing terms.