Stealth Visual
 

I'm not sure how much an addon can adjust, but I would like to see if this can be done: adding an overlay to take the color out of the game when my player is in stealth, ala Splinter Cell games.

To me, this color change is the best way to visually clarify that our character is sneaking, working far better than the eye icon or even assassin creeds limited attempt to grey out your location.

Can this even be done?

A semi-transparent overlay could be added to the screen, but we can't really take the color out of the game world itself. But maybe this alternative would work for you?

In an ideal situation, this overlay would appear behind interactable and enemies and the player, but in front of the environment. I don't know if this game handles sprites or layered graphics like emulators do, or if it's 'on top or nothing'.

If it IS possible, the implications for looting and resource gathering (an opposite keen eye, so to speak) could be game changing.

No, the game world is the game world. Totally separate from the UI overlay on your screen. The texture I described could be behind all of the UI if you wish it to be, but it would still be part of the UI, never part of the game world.