I know this isn't how emissive works, but I think that multiplying a constant 3 (or 1) by the diffuse and inputting it into the custom lighting would work. Or If I had an emissive map. I would use a constant 1 multiplied by a texture sample, then multiplied by the diffuse.
What Pior says... and further Don't assume that the lighting calculations are a simple multiply. They could be using image based ramp lighting in the screenshots you've posted, or the lighting calculations could be something other than a simple multiply by a scalar. There's no reason you can't mathematically bias the…
A few points from an artistic/illustrative perspective : - This kind of shading works best with two tonal values (and sometimes three, but with the third one being very thin). At the moment you have too many of them on your car. - It might be better to hand pick the two colors (light and shadow) manually per material, in…