I have an isolated environment, but am able to rsync data into my environment. So, I have rsynced updates from http://updates.jenkins-ci.org/ as described on the index page taht appears on the URL and have configures the Update Site value on the Plugin manager->Advanced tab to point to the update-center.json file in that rsync copy.

The problem is that all the URLs in the JSON file are for the real internet, not the host sering my rsync copy. It would be a clearer workflow if Jenkins would support replacing the bae portion of the URL in the .json file with the one configured in the update site so that I don't have to add special logic in my rsync process to create a custom copy of the update-center.json file with the URLs translated.