What is the control protocol for BrightSign players? How can I control playback from an external system?

There is no control protocol for BrightSign players. The active presentation determines the triggers to which the player will listen and respond.

This video shows the basics of adding interactivity (in this case, triggering actions via GPIO buttons).  The same concept applies to UDP messages (UDP Input), RS-232/TTL input (Serial Input), and other interactive events.