I tried this on an oled display (by removing the controller and accessing row and column lines directly).

Light certainly had some effect, but I didn't get to be able to have the display show something and act as a sensor at the same time (possibly because the capacitance of the row and column lines is too high)

Each pixel receives light from each point in the room, so almost all of them have the same value.

You need a pinhole to block vast majority of that light, so that each pixel on screen receives light from a different point on the other side of the pinhole.

Your OLED display doesn't have a lens though, so you would only see light differences

Maybe, but why do that when you can compromise webcams and display cables?

...if you work it for a while, you can image things in a radar like fashion. there was even a device that could be purchased that was a working television/camera, now obsolete...what you are suggesting requires physical compromise beforehand...