Source code ? You mean unrealscript ? I didn't found a good way to compute the screen brightness without the need to hook the framebuffer unfortunately (via DLLBind). However, once you have this data, you just have to use it to control the tonemapper inside the UberPostsProcess node of your post process chain. Regarding…
IIRC, Foyok did such a thing, and he had access to the source code, since it's easier to manage a dynamic solution like that on the fly rather then through Kismet or Materials. BTW, what solution base are you looking for?