It isn´t supported at the moment. But you can override the video behavior with custom css as a last resort. There are a lot of examples on codepen like this one, which should also work within bricks (if you change the classes accordingly).
just to let you know: we improved the background video option in Bricks 1.3.6 (which is available now) to display the background best possible. From now on, you can even choose an aspect ratio for your background videos that will help to scale them without squeezing them, so they look good in every scenario.
Please give it a shot and let me know what you think
I think this will probably be one of the underrated improvements from 1.3.6!
This was probably only one of a very small handful of features that I have felt was a little off (half baked). But now it works brilliantly like what you would expect of it from bricks!