Can you really register iMessage on an emulated MacOS these days? I'd love to learn more, the AIs I asked say it doesn't seem possible in VMs anymore.

I think you need to register on a real Mac (2 of 3 of my MBPs use OCLP), but then can use an emulated one if you add it to your Apple account. Either way, I don't recommend to use a protocol behind such a moat. Probably better to use Signal or Threema.