As it turns out, finding the shaders used to draw a HUD and actually isolating the HUD into a secondary render pass that can be blended back with the scene underneath it, is not even remotely simple 
SK already has features that allow users to find and share HUD shaders, for the purpose of taking screenshots with a game’s HUD removed. My recent thread for Baldur’s Gate 3, for example, was started mostly just to give the users a list of these shaders.
But that doesn’t do even half of what’s needed to make luminance control for the game’s HUD separable. I think I’m sadly the only person doing any research into this kind of thing, or there might be some real solutions.