I would love that feature too. I usually cast from iPad to plex-mpv-shim on my HTPC, cause I prefer to have touch library navigation with a minimal mpv UI on TV. This config is perfect for me, but the cast button is missing in the Watch Together mode. Also a lot of clients that don’t support Watch Together could get it that way via cast. I hope it would be implemented someday.
If you have votes left, you could create a feature request for that in the request forum.
+1, this is sorely missed at the moment. Being able to cast watch together would greatly shrink the gap for unsupported platforms as you can just slap a chromecast on for $30.