Did the G5 iMacs have VGA or DVI video out? Because if they did, there's no reason you couldn't put it in a custom case and use it something like a really piss poor excuse for a PowerMac G5 (piss poor in the sense that expansion opportunities are limited).

I have seen this done with a G3 iMac logic board after the CRT and/or PAV took a dump. It looked kinda silly, but *did* work.