And you could use it on a black screen without having and IR led if you allowed some individual pixels to turn on occasionally and flash faster than you can see. Whenever a finger is detected, more pixels can light up because you won’t see them beneath your finger anyways.

Yep, it certainly would.
Only disadvantage is that you’ll have to light some pixels to get a reflection on the neighbouring pixels.
This either means you can’t use it on a black screen/window, or you’ll have to use invisible light.
This can be done by using a 2×2 grid of subpixels ordered like R,G,B,IR.
The touch-detection could be done in the display-controller chip, which means the costs can be really low.

I’m not sure about the wavelength sensitivity for coloured OLEDs, but if IR-LED’s are more sensitive to IR-light, compared to other coloured OLEDs, you could start sampling when there is a change in IR-reception in order to save power.

All you’d have to do is scan the monitor in a travelling grid pattern every now and then to see if there’s any change, and once you find a finger or some other object, you can scan that area faster and more precisely to see what it does.

And the bonus trick is, that you can use a laser pointer as a stylus.

I’d like to use this but I don’t see it being cheaper than an actual touch screen lcd+controller part.