I've only found mentions of the firmware hack for G3 iMacs. So far I've found the hack for eMacs, G4, and G5 iMacs only.

Also on a side note, would a down converter from a 400MHz board with a 500MHz or 600MHz board? I don't think it would since the components appear only slightly different, but those differences that appear minor could turn out to be major to the system. :-/