@quizzical & others: I did some testing with the mini-media-player and the official integration join/unjoin features. It seems to me that the card is working fine as it is. I only did some basic join/unjoin testing and didn’t test all the buttons, but at least the basic stuff is working already.
However, it is necessary to modify the card config parameter platform from heos to media_player. Then it seems to be working fine. I realized this when looking at the code of the card here.
Example:
type: custom:mini-media-player
entity: media_player.heos_office
speaker_group:
platform: media_player # Notice this!!
show_group_count: true
entities:
- entity_id: media_player.heos_office
name: HEOS Office
- entity_id: media_player.heos_kitchen
name: HEOS1 Kitchen
- entity_id: media_player.heos_bathroom
name: HEOS1 Bathroom
- entity_id: media_player.heos_livingroom
name: HEOS Livingroom