I’ll publish an example for how you can do this relatively soon, I need to strip it out of an existing project.
What I do is I have a render target where I draw white masks onto a black clear. So on your GOs of whatever you want masked you need to attach a sprite with the mask shape as a white png with transparency. If you wanted some fading in the mask just erase parts of it. A black/white png with no transparency would probably work too.
Then you use the “mask”, with all of the masks drawn onto it, render target as the extra alpha in a shader to mask off areas of whatever you want masked. This requires masked objects to be drawn to their own render predicate too.
The final product is then drawn to a unit quad like other post processing effects are. If you follow the color grading tutorial you can probably learn enough to implement this yourself.