Hi, I recently managed to use my laptop as "bluetooth speaker" for my Android phone (via pulseaudio). While I can play/plause the current track using dbus (i.e. through the org.bluez.MediaPlayer1 interface), there is no user-friendly integration with Plasma. I suppose it's because bluez does not provide an mpris interface?
You should be able to control the player from Bluetooth applet, but yes there is no MPRIS implementation in BlueZ nor Bluedevil.
Oh you're right, I didn't realize I could expand the phone entry in the applet... ;)