Hey there. I was able to prove that it is in fact possible to post-process a UMG UI as if it were a 2D widget, however this only really works if your game is fully UI-based or 2D and doesn’t require a perspective camera (so… no 3D games can do this.)
For those who are wondering if it’s possible to post-process a UMG widget and make it look as if it were still in 2D space, I posted a tutorial over on the forums: Post-Processing a UMG User Interface - Programming & Scripting - Epic Developer Community Forums