I think a new 'rgbMod' shader command would be good. Similar to how tcMod applies an effect to texture coordinates, this command would apply an effect to the RGB values.
{
map textures/blah/blah/blah
rgbGen lightingDiffuse
rgbMod multiply ( 1.0 0.7 0.4 )
...
}
Would not affect or allow overbrighting, that is done by messing with the video card's gamma tables.