Hey!
I am not sure about which effect is the best! I myself would say that the one you got already seems cool! The idea of using a decal also is also good if you like something even better!
For communicating with the several cameras though I could offer you some suggestions! First of all you can bind your camera toggle to an event dispatcher and work your way through that!
Another even easier way would be to create an interface for your cameras (for the toggle) and then Get All Actors of Class! Then send your interface message to all the actors toggling the lights on/off!
post explains exactly that: .unrealengine.com/latest/INT/Engine/Blueprints/HowTo/BPHT_5/index.html