I seen someone requesting something similar so i though i would publish my solution,
Down For Maintenance
A simple little plugin to shut down a page for maintenance. Just drop the “Maintenance Mode” element on any page you want to have shutdown control over then you can either assign Yes/No for each page’s element or create a Yes/No “thing” in your database and point the elements to that for all round control. You can also insert your own custom HTML to make this your own style.
Default Page:
Control via Yes/No:
Control via Database Yes/No “Thing”: