I don't see how the material effect would harm anything - that's the
point of light prepass, allowing each mesh to interpret the light data
as it sees fit. The effect itself doesn't use any light data, but it
would look funny if every kart glowed in the dark, yes.
I was afraid we needed some light-specific information during the material pass; like whether or not the light was set to trigger backlighting or the light's color.
But actually I think you're right, we could just apply the effect during the material pass, for objects that are marked as such, and it would modulate the lighting we fetch from the light buffer :