 I think that's the best you're going to do...
Outlook doesn't do CalDAV and it doesn't auto-update .ics files and it doesn't speak the Zimbra calendar protocol, so as far as I know there's really nothing that you can do for an Outlook user who doesn't have the Zimbra plugin other than just share an .ics file and hope they update their snapshot regularly.
At some point Zimbra may expose an HTML view of the shared calendar to authorized external users, but that won't be happening in 4.5. |