Client Maintenance Drop-in
A drop-in plugin for putting a site into maintenance mode for every user except ME.
Instructions
The user ID defaults to 1, so if you are not user ID of 1, and you drop this php file into the mu-plugins folder, you'll lock yourself out and need to remove the file.
To use this file, drop the PHP file client-maintenance.php into your mu-plugins directory.
When you are ready to take the site out of maintenance mode, comment out the add_action( 'init', 'my_maintenance_mode' ); line from the file by placing two slashes // at the beginning of it.