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/).
This will take some effort to accomplish what you are asking for, but as a high level overview, you could use CTools' panel variants for `user/%user/edit` and `node/%node/edit` or whatever is equivalent in D6. And then you selection criteria as php code and then write some logic.

Not done this myself, but it should be possible this way.