I modified this plugin to allow site admins to access their site by changing line 124 from
if (is_super_admin()) return; //allow admin to use site normally
to
if (is_super_admin() || current_user_can('manage_options')) return; //allow admin to use site normally
Figured someone else may run into that same challenge.
http://wordpress.org/extend/plugins/wpms-site-maintenance-mode/