Resolved HDRP + MaterialPropertyBlock.SetVectorArray + Graphics.DrawMeshInstanced + Shader Graph

It sure is difficult to accurately find questions and answers to this problem. Maybe there is no such problem?
For what it's worth here's a report of actually getting
MaterialPropertyBlock.SetVectorArray and Graphics.DrawMeshInstanced working on HDRP with Shader Graph accessing the instance variables.
Unity 2020.3.17f1 + HDRP / Shader Graph Version 10.6.0 + Windows.

(This is not about Graphics.DrawMeshInstancedIndirect, or using MaterialPropertyBlock with SRP Batching)

It's straight forward enough to be usable, and I hope this method won't be invalidated too soon.
A custom function node sets up the buffer and retrieves the instance parameters like a standard vert + frag shader would, without the need for any UNITY_SETUP_INSTANCE_ID mechanisms.

So, the script side using MaterialPropertyBlock:

Code (CSharp):

1.  
2. // set instanceMatrices and instanceColors for your batch
3. ...
4.  
5. // apply instanceColors for MPB
6. materialPropertyBlock.SetVectorArray("_BaseColor", instanceColors);
7.  
8. // render batch
9. Graphics.DrawMeshInstanced(mesh, 0, material, instanceMatrices, batchCount, materialPropertyBlock, shadowCastMode, receiveShadows);
10.  

The shader graph side (what we used):
- Create a HDRP/Lit Shader Graph and a material using it
- Enable GPU Instancing on the material!
- Add a Custom Function node to shader graph, File: YourInstancePropertiesInclude.hlsl
- Add the outputs you need and call the function that retrieves the instance property you're looking for

In the example case the contents of the file:

Code (CSharp):

1.  
2. // Set up instance properties buffer
3. UNITY_INSTANCING_BUFFER_START(Props)
4.   UNITY_DEFINE_INSTANCED_PROP(float4, _BaseColor)
5. UNITY_INSTANCING_BUFFER_END(Props)
6.  
7. // Returns _BaseColor for this instance
8. void InstanceBaseColor_float(out float4 BaseColor)
9. {
10.   BaseColor = UNITY_ACCESS_INSTANCED_PROP(Props, _BaseColor);
11. }
12.  

Here's the part of the graph:

 

why is this not documented. thank you!