I’ve just improved a small but interesting plugin, originally created by BibLibre and then improved by @Daniel_KM : Maintenance.
It allows to show a “Under maintenance” message to not-logged-in users trying to access the public interface.
Message can be customized, and one can choose whether to show it inside a normal site page’s structure, or instead as a stand-alone centered message. As said, both interfaces (admin and public) will still be accessible to logged-in users.
Hope this helps!