How to extend PBR pipeline with custom instanced attribute?

[enbugger]

I want to slightly modify an entity mesh look that is rendered with StandardMaterial.

1. In vertex shader: slightly distort particular vertices towards per instance point in local space.
2. In fragment shader: offset its input texture coordinates for sampler based on a per instance parameter (e.g. offset on texture atlas). But I want to keep everything else so it looks like a PBR object (lighting, normals, metalness etc.).

I plan to have big amount to such meshes so preserving instancing would benefit the performance a lot comparing to having a set of uniforms and material for each entity.

However, I find it difficult to find any starting point.
It seems the new extension API shown in examples/shader/extended_material.rs only supports definition of uniform properties and not instanced parameters.
On the other hand, examples/shader/shader_instancing.rs shows the very minimal example of instancing which requires undefined amount of effort on PoC of integration all PBR parameters into it.

I would like to know how big effort can be to have the both features combined at current state of Bevy (0.12.1).
Or am I missing something and it is trivial task? (simple example would be ideal).