The above questions didn't have anything to do with the thread they were posted in, so I've moved them to a new thread.
The only connection to calendar sharing is that you want the absence to be visible, but simple free/busy will accomplish that. Or if you want to share calendars to everyone and have it mounted, that's an entirely separate topic.
Regarding your main question, I'm afraid I can't think of a good way to do this in Zimbra. You probably want to vote for
Bug 15061 – Support Workflow/Business Processes