It would be great if Cycles shaders had an input for light attributes. In addition to the Attribute Node there would also be an input node called LightAttritbute. You could then set attributes on light sources. When a ray reached a surface from a light source with an attribute the value of the attribute would be made available to the shader.
There are tons of applications for a feature like this. You could set a LightAttribute called EyeOnlyLight=1 for a light to bring out sparkly eyes without spilling onto the face. Or you could set a LightAttribute called BlackLight=1. The shader could add a totally different color (like purple) if that flag is set from the light ray.
The shaders would have to be clever to take advantage of LightAttributes, but it opens doors to effects that aren’t currently possible.
Please Comment as an upvote if this feature would benefit you.