A lot of plastic is already IR transparent but totally opaque in the visible spectrum. You can hack a cheap camera and take out the IR filter to make a neat "xray camera".

Maybe you should do that and write a paper about it. :)

My initial thoughts: 1) is there space underneath the key for such a sensor? 2) would the infrared light bounce off of the plastic on the key? 3) would transparent keys affect chicken-peckers (and other users) adversely?

Not saying it's not possible, although I imagine there will be challenges.