Shader post process material with attributes from a function in the source code

Hey weird question here.

I am trying to create a post process material that will map the illuminance values of the surface and create a gradient to show the lux representation of the scene (I am trying to make a daylight analysis tool).

My idea is to create a blueprintpure node with C++ to get the values I need from the results of a function in the source code and use a scene texture node in a post process material to map these values per pixel.

I am not sure if that would even be the correct approach, but I can’t figure out how to create a blueprintpure node inside of a post process material. Any help or ideas would be greatly appreciated.