Hi!
Your result is really good… I should give it a try 
Mi solution at the moment was to take the abs values of the VertexNormals and use each channel as UV for a band/ink textures, make a new float3 from the 3 TextureSamplesand then multiply it for (VertexNormal/abs(VertexNormals)) to retain the sign of the components.
This simply makes all the VertexNormals as discrete values so the surface can react to the light as if polygons share the same normals… the final look is very similar to a cel shading.
Pros: it’s very easy to adjust number,thickness and smoothness of the ink-bands… And of course it reacts to all lights in scene.
Cons:it gives me strange result somewhere because it tends to generate squared shaded areas.
I don’t have the engine at home but I’ll study your solution and try it on mondey as soon I reach the studio
Great job bud!