Wow. Do this wrong and you could be out a few hundred dollars.

I suggest you locate the local warranty service center and ask them if you can come by and read a service manual. There might be disassembly instructions there. And if that shop won't let you read the book, don't expect them to suddenly be generous and honor the warranty if you remove the screen to install an IR sensor.

How about a Xantech Dinky Link? While these ARE visible, I have gone into homes where I have installed them and forgotten just what I did, and it took me a while to locate the sensor. They really are quite subtle once you quit looking at all the details of the system...after it has been in and working for a while.