That feature would be useful for me as well.
My use case is somehow different. Due to the lack of an ability to remove preview branch (e.g. some work-in-progress version to show someone) I have another “disposable” site for that repo (to be able to delete the whole site if needed) and I would like to publish there only pages from branches disposable/*. Now I have to specify manually disposable/preview1, disposable/preview2 and/or reuse branches.