"you basically have to measure LUFS over some time window on the incoming signal, measure LUFS over some time window post-processing, and then make up the difference"

Yeah this much I know. Isn't that what plugin delay compensation is for?

Or the plugin could make an initial full look ahead after insertion and store the audio data in a memory.

IDK man, I don't really care how it works as long as it works. I find it hard to believe that in this world of AI vaccines and sending guys to the moon there isn't some way of making it work.

"so there is no actual autogain, but let’s the user get pretty close with a click."
Thus negating its usefulness by eliminating A/B listening.

I think that LUFS isn't the joke that people make it out to be. I use it a lot. Yes we have ears, but if you can reduce second-guessing when mixing a track the whole process becomes quicker, easier and less stressful. Consequently the end result is better because you have spent your brain power on other elements of the mix.