Injecting a games HUD (Pixel shader) after effects

A solution/workaround you think of would be more complex than that. A hud is usually not drawn all in only one draw. You also need to control when your binaries are hooked and that might be something useful before and after ReShade. Hooking before would mean ReShade is overlaying your draw anyway and hooking after would need you to identify not only the first hud ps but also the last.

If you still want to do that you need to build a library that fits the render API of your game (e.g. d3d9), do all the usual injector/hooking stuff and in addition to that detour all possible draw function of the API library. GeDoSaTo is dumping each draw on disk with the hash code of the previous ps (not sure about its retrieval in d3d9). While he is injecting his code after that hash comes up, you need to get all the UI draws on a rendertarget which you later use in your own shader which is applied AFTER ReShade as a texture to read from.