Add notifications to maintenance mode page?

Trying to figure out the best way to accomplish something, hoping someone here has an idea.

In a nutshell, when I put my Drupal sites in Maintenance Mode, I'd like to give visitors the opportunity to sign up for notifications when the page comes back up. Can be simple, something like a single textbox for them to submit their email address. Ideally, would be great to be able to have the notification sent automatically when Maintenance Mode is deactivated, but I'd settle for a manual email process for now if I could just capture the addresses.

So, any ideas? Only thing I could come up with so far would be to create a custom maintenance page, write the code for the form myself, and write a simple insert query into a yet-to-be-created table or flatfile, but I'm not a huge fan of that.

Thanks.

Justin