It has not been mentioned before, but I want to point out that you need to set the “Opacity Mask Clip Value” of the Material properties to something you can work with. For example. Currently in UE4 4.20, this value is set to 0.33. If you use the technique above with a translucent material set to 0.32 in opacity, the post processing material will not show.
In my case, i need to show only the outline of a completely transparent material, so I had to set this value to 0.0.