Moving on.
After some more experimentation I finalized the crown shaping part of the shader, settling on what looked like the most optimal method. It's an axis mapped superformula with only 3 (out of possible 7) adjustable parameters. This results in decent amount of silhouette variety and implicit fake 3D normals that can be calculated without too much cost. I also included some cheap fake AO, darkening the albedo if the normal is pointing down.
Now a bit trickier part. I'll try to add some semblance of clumping as it is one of the main visual features in reference illustrations. This will again be heavily simplified. Making voluminous looking clumps solely with a 2D shader is probably out of my shader wizardry league 🙂
So the idea is to use pretty much the same principle I implemented for stippling, just on a bigger scale. The silhouette is tiled with blobs representing clumps. They can be scaled and their normals are mixed with the main shape normal to retain the overall volume illusion.
A several layers of this are overlapped to make it appear more voluminous. This in now the final illumination base that will be used by the existing stippling shader.