I'm new the Zimbra so keep that in mind, but isn't this setup by default when you create a "resource" as the admin? At least I noticed when I created a resource for our conference room I was able to login as that "user" and share out it's calendar so everyone could see it. Later on I logged in as that user again and notice some mail had accumulated in the Inbox based on people accepting the share and testing setting up meetings in the conference room. I believe you could then either share out that mailbox or use filters to have any mail sent to that "user" automatically sent to another address.
As mentioned though I'm a newbie so it might be best to wait until someone can confirm/deny the above
