Ha, I was just reading some of the discussion and thinking it sounded quite similar to the JIT we use in our own (Linuxy) Time Travel Debugger.

It's similarly am x86-on-x86 JIT / emulator but (and I'm sure WinDbg's TTD is similar) most of what you want to do there is just code copying for any instructions that don't need special instrumentation.

And you want to run entirely in the cache of JITted code so you're close to native speed, rather than exit to C code and make decisions.