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.
I expect someone will eventually get around to reverse engineering the various M series specific instructions for qemu. Does imessage make use of hardware attestation to register with the remote endpoint?
Not really, you can emulate macOS on any Linux/x86-64.
But it is actually a good point to get a Mac Mini instead of a NUC. The Mac Mini is going to deliver better performance per Watt.