Using an FPGA makes it especially weird because a Z80 only has a few thousand transistors and would be pretty easy to just implement on the FPGA! That said, I'

Using an FPGA makes it especially weird because a Z80 only has a few thousand transistors and would be pretty easy to just implement on the FPGA! That said, I've used a microcontroller as a debug device for a retrocomputer and it made things a lot easier. But that's the way it was originally done -- using a bigger computer to build a smaller computer is "fair game".