I did not want to discourage you to make a layer system. I was just warning you that connection between a Layering system and material node editor probably can not be absolute.
It does not mean that partial/relative one can not exist.
Currently, Slots panel have 2 modes :
- Material Mode, listing all images used by current material
- Single Image Mode, to allow to paint an image that is not used by material but that is used to influence effect a modifier or to tweak particles.
We can easily to replace actual material mode by several material modes.
An absolute material mode working as current one that could list all images in material.
And a layered material mode that would only support nodetrees made of a principled shader + mixRGB nodes, with an admitted known limitation of not being able to handle all nodetrees.
So workflow would be : when artist starts to paint to prototype the asset, he would roughly use this layering system.
When he wants to become more technical, to insert procedural effects, to decouple things into different shader nodes, to emphasize some things with math/curve nodes : he stops to paint and modifies the nodes.
And then, to finally adjusts the image textures, he jump back to absolute material mode.
Your last post reminds me that there was also an old idea of supporting painting on a layered image format like .ora. All textures of material could be layers stored into one .ora file.