I don't think zimbra.com can address your problem. Maybe zimbardo.com?
For the out-of-office problem, we have someone whose job it is to record absences on a shared calendar. They are always entered as all-day events, even if someone will only be out a half day. Absences can always be found at the top of the agenda, in a distinguishing colo(u)r. They don't add to clutter, so there's little incentive to disable that calendar.
Optional public events are more difficult. Something we've considered, but not implemented, is an agenda@fqdn weekly digest containing not actual appointment invites, but hyperlinks to a web site that would inject the appropriate iCal into the user's calendar. No clutter unless the event is accepted, and no more work on the recipient's end than accepting a traditional invite. But a greater burden is put on the sender and the web programmer. One could imagine an agenda@fqdn address that accepted normal invites, posted them to a shared calendar, stripped iCal attachments, and dropped the text part of the message into a queue for a weekly majordomo/listserv/sympa/mailman digest, but that's a lot of code to maintain.  |