Poster Images for Video Backgrounds

A client is using a massive video in the hero image widget. Because of the size of the video, there’s significant buffering, during which only a black screen displays, and this is frustrating the client. While the obvious solution is to get the client to use a smaller video, it would be nice if when setting a video background we could specify a poster image as well.

(And, before anyone suggests it, just setting an image background doesn’t help all that much because the video element goes on top of it almost immediatelly and starts buffering.)