I honestly think if a vendor sells peripheral devices they should not be able to use private APIs between them and the host device.
Apple Watch should never have gotten preferential treatment, same with AirPods.
Your iStuffs should just pass the connection identified via the Bluetooth device MAC address or something and Apple support a shared device protocol and state.
That protocol is the magic, and other devices using it would not run the experience for the user. It would lose Apple money though.