Ah sorry, now i get what you mean :).
Ok. maybe not 100% procedural but my layers acts like that when i brush them and i can use still the good things like placing foliage only on grass or like phys material.
Dont know much about shaders but have you tryed a layered aproche? What do we get if we use layerblends in a function and input this in a non layerblended material? Is this possible? Will there be a 100% procedural material + Layer enabled for linking phys mat?