This works great! Exactly what I'm looking for. It did give me a popup asking if I wanted to open the app the first time I used the deep link, but I haven't gotten it since even if I force-quit Channels. Do you know what circumstances cause that confirmation dialog to appear?
In case it helps someone, here's the Home Assistant service call that I'm using:
service: media_player.play_media
data:
media_content_type: url
media_content_id: channels-beta://play/channel/6030
target:
entity_id: media_player.appletv_fr
This is so much better than the hacky solution that I was using on my Shield!