Yeah it would be nice to somehow be able to hook an alpha channel up to this media texture / projection.
You can multiply with alphas in U4 so maybe there is a way to do that.
You just have to render the alpha out as a duplicate of the movie but in black and white.
White masking the area you wanna keep and the rest falls away.
Just to get away from ending up with a square box projection 
Same deal with the buttons in the UMG.
Who wants a square button?
If I have the time I will dive into this and see if I can cook something up.
I will share.