There is really no need for a mod to do this as you can easy do this via your .htaccess file. Simply create a subdomain on your site (say, maintenance.example.com) and use .htaccess to point to it. You can do so like this. After your "RewriteEngine on" statement, add the following:

Code:
RewriteCond %{REMOTE_ADDR} !^123\.123\.123\.123$
RewriteRule .* http://maintenance.example.com [R=302,L]
This states that if your IP address is not 123.123.123.123, redirect to the maintenance subdomain. Be sure to replace "123" with your actual IP address.