"That said, I've only used a Ceibo emulator because it was provided to me for a certain project. In that project I had to use a certain I/O line--"

This is, indeed true for CEIBO and other non-bondout emulators. This, however, does only become a problem if you are not using external memory (otherwise P2 is address). I have just designed a P89C668 internal memory only and --

"but also being quite familiar with the tool you've selected to know its limitations."

-- avoided using p3.7 and p3.6.

If you spend the money on a bondout emulator e.g. Nohau, there is no such restriction.

Erik