you could experiment a little with a signal generator or a protected source of video signal, and try to "inject" a video signal, where you think could be a possibility to insert the signals... but take care, crt's operate at around 25kV, and can even maintain its charge when being disconnected for days. a good strategy would be to use an "insulating transformer", to keep away from line potentials against ground.
good luck on your hunt, and stay on the safe side!