Well, here is my semi-final solution.
I ended up using one of the spare door/window switches I have, since I'm not currently using them. But I do have a handful of doors and windows that still need them installed, so if I ever end up needing it, I'll either buy more or buy one of the other kind of contacts and replace this one.
The magnet is leftover from a 4-pack I bought of those cheapo stick-on alarm/chime things, dunno if you've ever seen them. You're supposed to be able to stick the alarm to the door and the magnet to the frame, and it gives you a simple alarm that the door has opened. I never ended up using it, but as I never throw anything away, I have now found a use for it!
I wasn't sure how I was going to affix the metal band, but then I saw I could slip it under the GDO's limit switch, and it holds just fine. Interesting...I WAS going to try and use the magnet in the carriage...but there doesn't appear to be one. At least, my door sensor would not detect one, and I certainly can't see one in the plastic..so I have no idea how the GDO limit switch works. *shrug* But nice thing about this solution is that I don't have to know.
While I was at it, my Dad had given me an idea. He said "Ya know, if you put a button here (by the garage door) then you could open it if you needed to without having to go over to the wall button.". That's not a bad idea, as our garage is actually pretty long, and usually what we did was open a car door, squeeze in, and hit the remote in the car. But instead what I did was, since I had used 22/4 wire to prewire for the door beam sensors, and they only need a single pair per sensor, I just scavanged the other pair and attached it to a momentary button.
Because it's going into my Datanab's analog inputs, and not a digital one, there's a built-in delay, which is good because I'd have put a delay in if it wasn't already there, so that bumping it doesn't make it go. So you have to press and hold the button. When the contact is sensed as closed, I trigger one of the relays on my Zeebox 250 for a momentary closure on the door opener contacts, and away it goes!
I wired that button, the "closed" sensor, and the "open" sensor all into the same 22/4 bundle of wires. They just all use the same ground. As long as they all go to the Datanab, then no problems. That does mean there's also a delay from the time the door opens until it is sensed, but unlike the front door, I don't have to worry that the door might be opened and closed before it's even sensed.
Anyway, thanks for all the help, I'm quite pleased to have this working.
