So I create a shader-graph based post process which then gives me a material to use as a FullScreenPassRendererFeature Pass Material.
OK fine - it works, though why can you not just add a custom post process instead of a FSPRF?!
But if I want to modify the effect at runtime it is a bit more tricky. So I can reference the original material in a script. This works, but of course the editor has this silly thing where your runtime modifications affect your actual original material 
. So although it works it is not ideal.
I could access the FullScreenPassRendererFeature in script also but I guess modifying that at runtime affects editor also?!