Lock out users during test case execution after upgrade

Customer has a requirement that there should be an option to restrict the Internal as well as external users to login Bitbucket until administrator carry out all the test cases after the upgrade or migration.
This feature is needed in situations like Bitbucket update, OS upgrade, GIT server version upgrade, DB version upgrade, and Java upgrade, etc., where Bitbucket functionality check is required post upgrade. Other users are not permitted to log in at this time until the Bitbucket administrator finally opens access for all users.