Question: While we do some server maintenance, how might be prohibit our users from editing or adding content but not put the site in offline mode. That is, we'd like to have content browseable but temporarily prohibit content from changing.

Answer: 
I have never used this module, but seems to do what you want: [Read only mode](http://drupal.org/project/readonlymode/).
I haven't done this (seems to be the common thread here!) but [Boost](https://drupal.org/project/boost) has a feature for running entirely without the database. It also has a crawler to ensure pages are cached. Note the warning! ![screenshot](https://i.imgur.com/ti8Bf.png)