Magento - Show a temporary page, still allow to use Magento

One simple task... I have a Magento store sitting in the root folder and need to show a temporary HTML page (that i've already created - [login to view URL]), while still being able to access Magento. Either by moving Magento to a sub-domain or by (preferably) getting something like the following to work: [login to view URL] (I have tried this, but it won't let me use Magento at [login to view URL] - it just redirects to the under construction page). I'll need you to either undo this change at a later date or let me know how to undo it when the site is launched. If it's just commenting something out in .htaccess or whatever that's fine. Thanks!