Puggo - Jar Jar's Yoda said:

If there were a way to have it choose pixels based on which pixel is less extreme (that is, pick the pixel closer to the midpoint, which should use the lighter source in the dark areas, and the darker source in the light areas - rather than averaging them), then it could probably be used everywhere. Hmm, I wonder if anyone has ever written a script to do that?

I actually think I've found a way to do exactly this in avisynth using only the mask,showalpha, and invert filters. Here are some pictures to demonstrate. It is an extreme comparison because I brightened one image considerably just to demonstrate what is happening.

Normal frame

Masked bright frame overlaid on top of normal

Bright frame

The image doesn't look great because I way overbrightened the bright frame to intentionally blow out the whites to show that it successfully only brightens the dark areas from the normal pic.

If you are interested let me know. For now though, I must do some housework or the wife will kill me when she comes home.