[nateroling]

Would this be a usable alternative to Plex’s Watch Together? Could I host a live stream of a movie and have friends watch it in sync?

[caladin]

If you can get the file to your friend, you'd be better off using syncplay. Basically, you watch the file natively in your own player like mpv, vlc, etc. and syncplay will manage syncing all of your pauses, seeks, etc.