Custom Maintenance Page for 2.7

I’ve waited until the official release of 2.7 to bring this up, but is there a way to customize the maintenance page? I’d like to put the date and time I started the upgrade, and how long I expect it to take. That way visitors know that they just need to wait like 30 seconds and refresh the page.

I searched the codex and forums, but I couldn’t find anything on this topic. Is it even possible yet?