"Beachcomber" wrote {howto sense open garage door w/o using sensor at bottom of door?}
A) Often you just need a short annunciation in some usually-occupied part of the house that the door is in motion -- if so, then hang a transformer on the opener unit's courtesy 'light-up-the-room' lightbulb socket(s), and run that low voltage across the house to a 12 volt lamp at said part of house. Sadly most modern units turn off the light after the door's been up for a bit, to save bulb life I suppose, so this isn't for just everyone's tastes. B) Don't sell jury-rigging short; for instance you can extend the 'reach' of a momentary switch with a long springy strap or whisker -- in the deluxe version, with a roller or slippery curl on the end. Have that bear against something at or near the top of the door, where nobody'll mind it. I just love little Cherry {TM} switches, and some of them come with this.
Or have a bracket on the door push on your switch on, or off to the side of, the frame via spring or something else spongy; the goal here is to make it all work with no precise positioning needed.
Where to scrounge scrap metal? Saw blades are nice and springy, and you can savage discarded mattresses for good stout coil springs to hacksaw a few inches from...
Last time I was called to 'fix' an old project like this, it was actually an erratic bulb filament at fault, so try a new remote-socket bulb _first_ when things get flooky. Glue spare bulbs to your fixture with rubber cement, as the next homeowner ten or twenty years later won't have a clue where to buy them...
C) Drilled concrete probably isn't as horrible a liability as one might suppose, though it's rough to accomplish. And they make some really wonderful glues nowadays, so you could put stubby screws in to plug the fixture holes, and glue the base down instead... Chisel it all off cleanly if you have regrets later.