I already reached the limit of what I can do in terms of post material effects because I’m limited by the few render passes I can use. There is Custom Depth which is nice to use for own things but that is basically already it. There is also the Decal Mask render pass that can be used differently then its purpose when no decals are used in the project but there the limitation is that Set Decal Mesk on actors can’t be changed at runtime. I also used the transparency pass (which is currently PostProcessInput1 when active in the project settings) for something different then its purpose so I could realize the main effect of my current project.
The best way would be to give the user unlimited access to render passes, so we could add our own Scene Texture types and then use that in our post materials, a additional tickbox for it could then become available in the components/actors rendering settings. Even better would be a custom material input that then gets rendered in the custom Scene Texture. So there could be lots of masks with just a single render pass simply by using it colors. Like color 243,123,012 is a mask in the post and color 123,231,031 is a different mask, etc.