I’d suggest looking into the UMG viewport widget.
It is still tagged as experimental, but has worked find when I’ve used it.
You could combine that with a simple black overlay that fades in behind the viewport widget.
Alternatively, I believe the latest iteration of the scene capture components (4.13) allows you to specify which actors to render and which not too.
If so, attach a scene capture component to the current camera and tell it to render the actors you want.
Next, pump the resulting texture to an image / overlay widget, along with the simple black overlay mentioned earlier.
That’s all I could come up with off the top of my head.
Hope it helps.