I am missing the media_player.play_media service for my Linn device, too.
At the moment I am using a workaround to trigger a radio station on my Linn device:
There is already a Roon component for Home Assistant. In Roon I have defined the needed radio station. From Home Assistant I trigger Roon to play the radion station on my Linn device.
I cannot seem to get this to work. I have an OpenHome media player and I can control it fine from HA (change volume or track, play/pause), but play_media returns a AttributeError: 'NoneType' object has no attribute 'ControlUrl'. I’ve managed to get it playing the current playlist via the media_play service.