Confirmed that this works! Setup: Tailscale on android phone, Chromecast Ultra without tailscale. Edit settings in Jellyfin app to play on external player. Choose VLC as external player and then cast from VLC to Chromecast Ultra. Caveat: casting a stream with subtitles does not work.
How do you cast from Kodi to Chromcast, with a plugin? Also, do subtitles work correctly when casting?