Installation:
extract system+sdk folders into your AC installation, does not overwrite anything.
To use, select ksPerPixelMultiMap_emissive in ksEditor. Paint your new emissive texture with the brightness and colour of light you want from it. Full white = original emission.
To redistribute, include the system folder with your mod.

Potential uses:
running lights (as above)
buildings on a single material with just the windows lighting up, not the entire texture
windows lighting up various amounts across a building, instead of all identical
backlit advertisements that don't light the entire thing uniformly

Credits:
Stereo