I played with the web video remote again and gave it a few more things to do. In addition to Play and Pause, it can slow the video down, speed it up, or jump back ten seconds. Sometimes rewind is the most useful button on a remote.
It still uses a very small PHP mailbox rather than a persistent socket connection. The remote writes a command to a file; the player checks for one every 300 milliseconds and the server replaces the consumed command with a no-command value. JavaScript maps those strings to play(), pause(), playbackRate, or currentTime on the video element.
This is not a multi-user remote-control service. There is one shared command file and one intended player. For an experiment, though, it is nice to keep the moving parts small enough that I can see exactly what each request is doing.