I believe the glow map is a separate texture, but I'm not sure if you can change it via scripting. For items that have a proper premade on and off version, it'd probably be easier to place both in the same place, set the "off" version as the enable parent (opposite state) to the "on" version and the actual light, and just disable/enable the "off" version to get your switch. No scripting necessary.
If a fixture has no premade "off" version, you may have to edit the .nif in Nifskope to remove the reference to the glow map.