Support for casing to Apple Airplay devices.
Playing music using a existing Apple Airplay infrastructure (speakers) using an (unrooted) android device.
Support to the Apple community.
I am not aware of any possibility to stream from a subsonic compatible server to Airplay devices using a non rooted android device (e.g. phone).
In some other post you mention licence restrictions by apple and the issue of the encryption key. As far as of my knowledge encryption is not necessary for simple Airplay cast (playback), there even exist some libraries (GitHub - openairplay/open-airplay: A collection of libraries for Apple's AirPlay protocol) that do so. Capturing the audio signal on a unrooted android device is not possible, thats why your app would need to directly support it.
Most likely the android device would need to act as a on the fly aac encoder and intermediate streaming server. This would limit its usage to the local network, but would be a non-issue for me.
I think I explained my idea. Thank you for your efforts creating this great app. I really love it and happily pay for it. Let me know if I can be of any further help.