How do I make spikes show wherever you would see solidify lines with flipped normals?

Question

Using Cycles, for a cartoony animation character I need spikes to be seen only from the character's edges and behind the character but never seen in facing angles. I want to avoid having to render the scene twice to be composited after.

For example: 2 meshes, 1 skin color, smooth, rigged, 1 blue spiky mesh sticking out, rigged, never seen in front.

I have no idea how to accomplish this.

Do I have to account for camera angle and normals of the spike's base?

Can I just layer one over the other with nodes?

Default look:

Approximate Desired result done in Photoshop.

Answer 1

This seems like a task for the compositor, but you explicitly stated that you don't want to render multiple times. That means only AOVs remain an option for compositing. After trying it I discarded the idea, because it will cause artifacts. Instead I found a solution using only the shader and some Custom Normals.

First you need to redo the spikes. You need a spike geometry with Custom Normals pointing along the spike direction. You can do that in Editmode of the spike by using Mesh > Normals > Point to Target or using Alt+L. Then you can use instancing to put the spikes at the vertices of your model. Then you need to disable ray visibility for any other ray than a camera ray in the object settings of your spikes.

After this setup you can make this shader:

The result looks like this:

You are also free to change your material in any way you want, which could have been difficult with the compositor approach.

Here is the Blend File:

Answer 2

The closest I got was using cones with inverted normals and backface culling enabled:

But as you can see, cones pointing slightly towards the camera still get shown.