Every single other thing on the machine, including the ROM, is expecting a PPC, not a 68k or a Coldfire. Won't work, IMHO.
I could maybe see it working as a PCI card with a set of INITs that patch the ROM to trap all 68k code and redirect it to the Coldfire with some re-interpretation along the way for incompatible opcodes. Maybe.
But then one really does have to wonder why you'd bother when it would be a whoooole load easier and cheaper to stick a 1 GHz G4 in there and run 68k apps on it.