I know decay can be added to an actual light source, but I'm unable to use lights in this project. Instead, I'm forced to use materials with luminance.
Said materials with luminance make other polys cast shadows. Which is all well and good, but they also cast shadows on polygons which are quite far away from the material with luminance.
Is there any way to control how far the light will 'go' on luminance'd materials?
I don't think there's any way to control the falloff of luminous light sources, but you could adjust the "receive GI" percentage in the materials of the objects that are receiving too much light. Another option might be to use a plane to block the light, with "seen by camera" disabled in a compositing tag.