I can only imagine that "burn-in" or "pixel wear" compensation is done via signal monitoring and pre-programmed look-up tables. No need for pixel measuring circuits IIRC.
Non-uniformities originating in the TFT is another story. Compensation circuits are needed and are costly.
This is all from my memory from a few years ago so things may have changed.
Given that what has to happen to translate a pile of bits into a discrete voltage (a digital amp basically) for each sub-pixel, measuring the line in and out can't possibly be that complicated. It's not as though this would require 24 million chunks of monitoring electronics. I just can't imagine this being difficult given some of the memory controllers I've seen. In the case of a TV display, I believe you basically have an address (a coordinate system for the subpixel), a voltage, and measure the results of applying that voltage to that address.
I'll have to see the design details to know for sure what's happening.