OK, so I’ve checked it myself and here it is. The API (undocumented) to play a musiccast playlist is:
<BaseURL>/v1/netusb/manageMcPlaylist?bank=<playlist_number>&type=play&index=<track_number>&zone=<zone>
There are also a few others, you can check below for REST API and example responses from my system.
GET /YamahaExtendedControl/v1/netusb/getMcPlaylistName
{"response_code":0,"name_list":["Playlist 1","Playlist 2","Playlist 3","Playlist 4","Playlist 5"]}
GET /YamahaExtendedControl/v1/netusb/getMcPlaylist?bank=1&index=0
{"response_code":0,"max_line":11,"bank":1,"index":0,"track_info":[{"input":"server","text":"Hello","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"Send My Love (To Your New Lover)","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"I Miss You","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"When We Were Young","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"Remedy","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"Water Under the Bridge","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"River Lea","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"Love in the Dark","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"Million Years Ago","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"All I Ask","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63},{"input":"server","text":"Sweetest Devotion","thumbnail":"http:\/\/10.144.1.162:32469\/proxy\/df5def65beafa7527493\/albumart.jpg","attribute":63}]}
GET /YamahaExtendedControl/v1/netusb/manageMcPlaylist?bank=1&type=play&index=0&zone=main
{"response_code":0}
Updated now also on my repo
Edit:
BTW, I’ve also found this repo, which includes (in the php files) above as well as some other APIs for MusicCast.