I was thinking in something similar since years ago.An Amiga accelerator using an x86 cpu as emulated 68k.The x86 is used 100% only to emulate 68k instructions and you used the rest of the logic of the accelerator to wire to CPU slot.The Amiga only see a super fast 68k cpu and the x86 are always emulating a 68k and only see and returns 68k code
You can even use the newer ultra low power consumption neetbooks x86 cpus for using in the trapdoor slot of original amiga cases
Said this, I would add that the cpu could be of any family, we are talking about x86 but an accelerator with the same concept could be made with ARM, PowerPC,MIPS, SPARC or with a FPGA
