On an intel based machine PPC emulation is all but not worth it. If it was desired PearPC emulation cores would have to be the only viable options but with a PowerMac or Pegasos classic based ppc upgraded emulation may be a way to run AmigaOS 4 at a high speed.
I imagine it would be similar to running the Fusion mac emulator to run Mac 68K apps at nearly full speed but in reverse.
Do any of the more proficient hardware devs have any idea on how much work would be required to an emulated pass through PowerPC cpu to the mac version of UAE?