As described in the title, it is currently impossible to load a post-process shader that is not located in the minecraft namespace.
Even when attempting to load a post process shader json file directly, any pass names are immediately assumed to be in "assets/minecraft/shaders/program", and trying to specify a namespace with:

{
  ...
  
  "passes": [
    {
      "name": "namespace:id"
    }
  ]

  ...
}

results in minecraft attempting to load "assets/minecraft/shaders/program/namespace:id.json", which is not allowed and causes an exception.

The same issue occurs when attempting to specify a namespace in the program json file.

Solving this issue should result in better vanilla post-process support for resource packs if so desired, and if not would still result in much better mod compatibility as there are no extra mod libraries required.