Pretty much it’s the Beeper devs and one other. But the initial setups were really nothing more than using a Mac on the backend with a an adapter to Android.

Beeper and one (maybe two) other were pretty effective at it.

Beeper Mini is a different thing altogether. It uses a service to translate ANP (Apple Notification Protocol?) to GCM (Google Cloud Messaging), which are the respective notification handlers.

The Android client is able to comm directly with iMessage servers, unlike the original Beeper and the other ones.