Making the gamma adjust up or down based on the value of a few pixels seems pretty straightforward, so I assume there's some good reason it's never done. The best I've seen so far is Giants, which is mostly big sun coronas.
Still, it seems simple enough: take a few pixels from different parts of the screen, average the brightness, and gradually turn the brightness up or down. Depending on how often you updated and how many spots you used you could make it pretty gradual and smooth.
Is therea simple way to grab the brightness of a single point?
Can the gamma and brightness settings be changed from a mutator during play?