BillHarrison,
What you're talking about is known as an in-circuit emulator (ICE). We used them where I worked years ago for OS development with processors such as the Z80, 6502, etc. However, given that modern CPUs practically have debugging built-in, I don't know if ICEs are even used anymore.
Anyway, your idea could possibly work assuming you can get the timing issues worked out, but what's the advantage? You have an old Amiga using a PC as a very fast CPU - then what? Don't all your 68K programs work fine as-is on a real CPU? And for those that you want to run faster, don't you have better software on your PC to do those tasks anyway?
Technically it's an interesting idea that may have some merit, but from a practical standpoint it's not worth doing. It's not like you're going to gain anything by running Lemmings faster, for example.