Given that Zimbra 8 is out, and it has retention policies for tags and folders, this is of limited value. However, I cleaned up and posted a script I've created for purging email from Zimbra 7. Any query that can be used in the Zimbra search box can be used to find emails and delete them.

https://github.com/straydogstudio/zimbra_purge

If anyone is stuck at version 7, this may be useful.

Noel