I just wanted to say that I used your "trick" (thanks for the tip) with zmmailbox and wrote a C# windows service connecting with ssh to my zimbraserver, autoupdating my feeds with a given interval (different for each feed.) Just in case someone have any use for it I've written a blogpost about it and uploaded the code + installer:
Onkelborg's blog: Zimbra - Updatera feeds / Update Feeds