Currently, I do not believe there is any way for us to access the value of the grass paint layer within the grass material.
I would love it if this value could be exposed to the instance based on the position where the mesh is spawned.
Having this value could expand the usability of the grass node tenfold.
Imagine it this way, you can immidiately use the value of the Alpha to shorten the grass mesh height as you transition from a grass patch to a dirt patch.
The data needed to do this is already in engine, we just do not have access to it.
it should be made avaliable like the Per Instance Fade was.
It is a very similar idea, its just derived from a different/pre-existing value that the engine should have access to when generating the grass maps.