After getting the extra RAM to work, I added an EEPROM as well. After which my PCW started randomly crashing. It's taken several days to get to a state where it works reliably, and I think I'm there now. There were several mistakes I'd made. For a long time I thought it was the breadboard's fault, as when I built my homebuilt Z80 computer then I had a bunch of dodgy breadboards which didn't make good electrical connections. I'm not totally sure this one is fine, but I don't think it's the main problem. I wondered whether I was drawing too much power, although this seemed unlikely. I hoped I hadn't blown up my PCW, but disconnecting my external circuit proved that I hadn't. After fixing a few silly mistakes in the VHDL code and still not curing the crashes, I tried leaving MDIS floating when I wasn't disabling the internal memory, instead of driving it high, and that seems to have worked.

Anyway, I've managed to copy some data to the ROM and have it still be there when I switched the computer off and on again:

Now, unless it starts randomly crashing again, I can get onto the fun stuff, which is making the PCW boot from the ROM, and then adding an SD card.