Send a regular reminder (or do other normally cron-dependent tasks) while in maintenance mode
I, personally, can think of a hamfisted, hacky way do do this, but I would prefer for someone else to provide a more elegant solution.

My web host doesn't allow SSH, but does have a "tasks" feature that essentially pulls a designated website URL at regular intervals; I'm currently using it to run Cron externally and get around Drupal 7's default Poormanscron implementation.

So I could bootstrap Drupal from a custom script and have it get pulled as an hourly task. It would variable_get('site_offline') and do nothing (or maybe redirect to index.php in case a 'bot discovers it) if false and invoke the Rule that sends the reminder e-mail if true.

Heck, if Rules or Drupal's e-mail abilities are also disabled while in maintenance mode, I can just generate the e-mail with straight PHP.