I don’t actually need the pigment map to look good in untrusted mode: I can have the material itself provide a fallback texture! There’s nothing stopping me from putting a
_MainTex slot on the shader which just does nothing on the shader, but which would be used by the Standard fallback; I can use other texture slots for the pigment map and palette. So obvious.
Also, when laying out the palette, it would be helpful to group related things next to each other, which makes mipmapping play more nicely (and also makes authoring eaiser, although still not as easy as just fiddling with sliders in the Unity editor, unfortunately).