Great to see this feature. Unfortunately it has no priority. All elements are shown like multi row header that needs to be created via sections to have a sticky header (bricks_before_header). Popups on scroll appear and so on.
It would be great if Maintenance template would have an above all priority. Also an exclude by IP would be awesome to test pages on front end on different browser without exiting the maintenance mode. I have function for this but would be great to have it in the box.