I've often thought about creating a CPU substitute for these older machine. For the most part, we rarely do development of Z80 and 8080 machines, we are mostly diagnosing and repairing these machines. For diagnosing, we rarely need to run the original code on the machine. In fact, the original code is a poor diagnostic tool. It rarely repeats a sequence but once on reset that we need to analyse. Any need for an ICE to develop a machine can usually be done with a ROM ICE ( a much simpler setup ). We have cheap controllers that run at 72Mhz or greater that can run I/O fast enough to run diagnostics on a 8080 or Z80 bus. The interface would be mostly buffers. No additional programmable devices are needed.
Dwight