So we know that Zimbra currently automatically publishes your freebusy information in a form that external Outlook users can use:
But why stop there? Shouldn't you also be able to enter a location for the freebusy information of a Contact you've created in your Zimbra account, and then access that when setting up a meeting with that person?
(Even better, if this could somehow be relayed to CalDAV clients...)
Is there an RFE for this feature?
I believe that the work-in-progress extensions to the CalDAV standard and/or CalConnect are supposed to include standards for cross-site freebusy access. I don't know if the method supported by Outlook is going to be the model, although it's straightforward enough and seemingly based on open standards. But including this in Zimbra now would be a great leap forward not only in collaborating with outsiders who use Outlook, but also with other Zimbra sites.
(It'd be nice if a published full-detail iCalendar file could also be used to generate free-busy, as this would let Zimbra users work with even more outsiders using e.g. Lightning/Sunbird or iCal.)