Custom WordPress maintenance mode that allows super admins access to the Dashboard

s-maintenance-mode.php

<?php

add_action( 'init', 'x_maintenance_mode' );

function x_maintenance_mode() {
	if ( defined( 'X_MAINTENANCE_MODE' ) && true === X_MAINTENANCE_MODE ) {
		if ( is_super_admin() && is_admin() )
			return;

		die( 'Site is currently under maintenance' );
	}
}