Upgrading a large multi-site

We are about to upgrade a large multi user site from MU 2.9 to 3.1.2. We would like to be able to put the site in some kind of offline mode to prevent anyone making changes while the upgrade is happening, and also to display a global message to all visitors to any of the blogs. Does anyone know how to do this?

Update: Minutes after posting this I discovered this post http://sivel.net/2009/06/wordpress-maintenance-mode-without-a-plugin/ about creating the .maintenace file and maintenace.php. This is exactly what we needed so we're good.