The display processor would have to have some way to turn off the backlight LEDs and then sense the voltage generated by the laser. It is unlikely that the signal would be able to get back through whatever power device controlled the backlight power to get to a processor pin. The rest of the LED strings would probably load the signal down.
You wouldn't need to turn the backlight off, but yes, you'd need to do complicated processing on the display CPU which is already at its limit doing screen ops.
In practice the backlight would be off because the input/output pin on the controller would need to be switched to the input mode from the output mode. If the backlight was driven, that drive current would swamp out the current induced by the LED(s). See Appendix E [1]. This attack only works in very specific circumstances.
It would be possible to measure it while on with the right hardware. It's unlikely, yes, but chips are moving to more generality and it is kind of reasonable that we would see such overdesigned chips at some point (analog + high density IO for screen in same reusable package).
