What make and model of garage door operator are you using?
Some have facilities for external status monitoring, or for connection an optional adapter to do that - eg. the Hormann one I have has a status interface unit available that has relay contact outputs for open & closed, which I've got connected in to my home automation system.
If yours has something like that, it would just need both relay normally closed contacts in series to drive a strobe or beacon.
Thanks for that... that did cross my mind that the installer (who already did a most basic installation) may have such an add-on option in stock.
It is just a matter of time for me to see them again (as they are still waiting for me to finish wiring up the yard to test the controls from inside the house). They already tested it at the motor box outside directly without any wires coming from the house yet. I have been spending every day digging, cutting electrical tubing, bending pipes, and pulling wires thought the yard as fast as I can to then call them again and repeat the yard test from inside the house.
What they installed has only limit settings to stop motion at the limits they set it at, no warning lights, no obstruction sensors, nothing else.
I have purchased 110V AC LED light strips to go down along side the garage door rails (which look awesome!) and thought I could later find a way to make it blink by the time I'm done connecting up all the wires (for the motor, yard outlets, street lights, yard lights, future water pump, etc.). Getting stuck lead me here.
So, in short, I either call them ahead of time and inform them that I'm still delayed in wiring the yard (!!) and want to know if they sell a warning light module, or we press on independently and maybe learn a thing or two. I'm game either way.