This was a random thought I just had. I was reading a bit into some things here
about using the GPU as something of a co-processor. Would it be at all possible, no matter how difficult, to send off the MMU requests to the graphics card, provided it had a powerful enough GPU? Let the CPU deal with the PPC-x86 translations, and use the GPU to calculate all the MMU conversions. I need to read a few more books on modern PC architectures and programming languages, but something in the back of my head says this is concievable.
Even if it were possible, you'd be far better off letting the native MMU of the host handle most of this task for you... Memory conflicts between reserved areas of the host and guest OS aside, I think some of the only problems you'd have is limiting the page size to that of the host OS, and when emulating MMUs that have no-execute protection on hosts that don't, you'd probably only be able easily protect pages that haven't been re-complied yet.
That said, I wonder if any available GPUs would be able to pick up some of the slack in altivec emulation.