How would an open implementation be compatible with this, given that Apple's implementation is based on an Apple-operated PKI?
Note that this is only a conversation about sender identification, which allows sending to a "non-world-visible" receiving device and confirmation-less sending to devices with the same iCloud account on them. Anonymous sending isn't cryptographically gated by Apple, to my knowledge.
That still requires you to have (access to) a Mac and an iCloud account.
It might be possible to reimplement the required Apple API, but as demonstrated by the iMessage/Beeper saga, they usually shut such things down pretty quickly.
Note that this is only a conversation about sender identification, which allows sending to a "non-world-visible" receiving device and confirmation-less sending to devices with the same iCloud account on them. Anonymous sending isn't cryptographically gated by Apple, to my knowledge.