Comment by Wowfunhappy

Would something like https://github.com/matrix-hacks/matrix-puppet-imessage have not worked?

I'm almost certain I've seen other implementations too, but I can't find them now.

Edit: Oh, here was another one: https://github.com/RomanScott/weMessage It's for Android, but you could presumably reuse the server implementation to create a Windows version.

Edit2: And yet another one, fwiw, but this time it's Android-only and closed source so less useful. https://airmessage.org/