I think I've come up with a relatively straightforward way to provide this. I'd like to propose the following:
- For Lamps that are connected to a circuit network, add a text field to their editor UI where you can enter a custom message. The message should be blank (no message) by default, so the initial setup is exactly like Lamps are right now.
- When the Lamp is on, display that message in the top left corner, like the way mission objectives are displayed.
- When the Lamp turns off, get rid of the message. Make a small time delay before getting rid of the message (maybe 2-3 seconds) so it doesn't get crazy if the lamp is flickering.
- Lamps not attached to circuit networks should never display messages, because that would be useless (and these lamps don't have editor UI's anyways).
- Messages should stack, so if multiple messages are visible at once, you see them all.
Of course this could also get its own entity. I'm not married to the idea of using lamps I just think they naturally work together. If it had its own entity it could be part of the Circuit Network tech (which btw is the same tech level it'd be available at if it were part of lamps too, since you need circuit wires to have access to the lamp UI).