How to make custom login URL accessible in maintenance mode
A viable solution for making the path/URL/page available while in maintenance mode is the use of hook_menu_site_status_alter().

This hook is called after checking whether the site is offline (in maintenance mode) but before the current router item is retrieved and executed.

Code below should be used in a custom module:

function CUSTOMMODULE_menu_site_status_alter(&amp;$menu_site_status, $path) {
  // Allow access to my_module/authentication even if site is in offline mode.
  if ($menu_site_status == MENU_SITE_OFFLINE &amp;&amp; user_is_anonymous() &amp;&amp; $path == 'custom-path') {
    $menu_site_status = MENU_SITE_ONLINE;
  }
}