Ah, yes, that makes that error go away.
I went back to basics, tried to put a simple diffuse shader (no texture maps, nothing but a color node plugged into diffuse) from shader forge as the material for the shaderAtlas. No go at build.
I've been hacking at the code trying to strip the shader forge code down to see what makes it unhappy, no luck so far, quite a bit more sophisticated shader declaration to create a simple diffuse then what I'm used to.