I have tested this bridging app and while it works there are a couple of quirks I have discovered.
The first issue is that once you install this app it prevents you from selecting alternative music apps that is available on Android Auto. For example if you select Deezer it will present the Poweramp Auto app instead.
The second issue is that the Poweramp Auto app disconnects the bluetooth audio after a few seconds of stopping play. Before this I use to be able to play a music app then hit stop and the audio would still continue to function if I manually ran an app on my phone that outputs audio such as a stream.
Only when I uninstall Poweramp Auto do both of these issues disappear.