Since Beeper has stated that they're done if Apple doesn't like their latest solution, what are other options? Two I know of are AirMessage¹ and Texts², both of which seem to support this in a way that doesn't bother Apple.
AirMessage requires you to run a server on macOS, and Texts only supports iMessage on macOS. That's why Apple doesn't mind, because neither of those services is hacking iMessage itself.