- Node tree should not be a part of the output of the shader node.
1.1. You can not use material node tree as read-only asset and with custom NPR tree.
1.2. You can not combine NPR and Shader tree assignment in geometry nodes.
1.3. You can not share the same material with different NPR in the same project.
New nodes have to be or part of the graph or be in the tree that is not part of the graph. Zones is the way to make pseudo graph so nodes keep in only one tree but can be in other execution context.
You can put NPR ID selector into additional list of IDs just like materials are. - I do not see the point of the NPR, sorry.
a. So this is just composition with by-mask implementation (node tree with effect is assigned to some certain part of the mesh)?
And its worth to do not just add new way to put mesh selection into compositor?
b. Or this is actually change the way how render is happening (something like custom ray reflection or global illumination emission with toon effect)?
But why this is eevee-only (as i get that) and not actually cannot be used multiple time in combination with shader?
c. Its just way to do not save some temporal layers with info?
But why its is worth to add new whole nodal system, instead of just unified interface of outputting additional layers of render info? And do this for cycles, eevee and custom engines? - Image sampling should only be an explicit with implicit screen space UV input. Not sure if current version only a temporary, but just to make sure. It is possible to make ID data type compatible with multifunctions and operate over them in the Functional nodes which is will be compatible with all built-in backends. So in the result version new node tree should not introduce this implicit conversion between Image and Color data types.