@roslof Thanks again for your help here with testing. It's much appreciated.
It looks like .RVZ support got added in dolphin-emu-5.0-12188:
https://dolphin-emu.org/download/dev/99822518994d62def4e336f6746d6a2e2d108fb2/
I will spend a little time and see how doable it might be to backpatch the changes of 5.0-12188 into 5.0-4544. I can't promise anything as 7,000 commits is a lot revisions where major structural changes to the code may have occurred in that time that could make a back-patch very difficult. If this were the video backend or JIT cores I would say there is no way I could roll-forward the code that far. But we might get lucky if the IO core is a more sparsely maintained section of the code and that there aren't too many changes. I will give it a try and I will let you know if I make any progress implementing rvz support into this old version of dolphin.