Thanks to Casino Royale, no waist treatment has become an acceptable style element in black tie apparel. I'm personally not for it. However, a true waistcoat feels overly formal for most of today's black tie events where half the attendees are certain to add "optional" behind the black tie, regardless of the host's intent and come dressed as a mortician or worse.
But going a different direction, the cumberbund still brings to mind images of the 80's and their matching pink cumberbund and bowtie ensembles.
So I was thinking what could look pretty current and sharp, while still respecting the rules of black tie, is a flat front cumberbund (i.e. no pleats). I imagine it as just a bit narrow