I've now done a bit of research on this, and it seems
some other video players are allowing control of the
brightness for playback through VMR (Video Mixing
DirectX 8.1 & 9 offers VMR interface, and players like
ZoomPlayer, Sonic Cineplayer, NVDVD, can take
advantage of it to give full color controls.
Perhaps you can look into this, it really would make MPC
very complete. Keep up the great work!