How to synchronize QuickTime video playback with 3 different Macs?
I don't know how well synchronized it would be, but you could try using at (or launchd or cron) to run an osascript command at a specified time.

sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist
at 16:12 &lt;&lt;&lt; 'osascript -e "tell app \"QuickTime Player\" to play document 1"'