The documentation for Gamescope has now been updated to note it supports a “subset of Reshade effects/shaders” and so this provides an easy way to layer “shader effects (ie. CRT shader, film grain, debugging HDR with histograms, etc) on top of whatever is being displayed in Gamescope without having to hook into the underlying process”.

To be clear, this functionality isn’t available in SteamOS yet but hopefully it will be coming soon.