Aaaaah hahaha turns out SteamOS' gamescope does exactly that. It hooks into Wayland and can apply a subset of ReShade shaders to whatever it's running on top of, which might just as well be the desktop, because why not, it's just another program, and it's your computer, you can do with it whatever you want.
You won't believe what I now will say, for the first time in 30 years of computer: This begins to look like the #YearOfTheLinuxDesktop.






