Viewport specific materials?

I've been wanting to find a way to easily do render passes but also capturing these render passes in a viewport. Sure we can add extra passes to shaders, but as far as I know there's no way to tell Godot to only display that extra pass in a specific viewport.

This question has already been asked before but never got any answer: https://godotengine.org/qa/45262/render-material-shader-only-to-specified-viewport

I thought, since I want to know too the easiest way to do this, that I would try and signal boost.