What I think could work would be to provide the bare WebDAV functionality (not sure if Zimbra does this outside of CalDAV, which is an extended special version of WebDAV) and emulate the data format that Exchange uses to share calendar data with Entourage. I believe it's written in XML; I came across someone out there at one point who'd written something to parse the Exchange output and read it into iCal. So to provide Entourage support, you'd do the same thing, sort of in reverse.
If Zimbra included this it would indeed be welcome.
EDIT: Here's the info I found on "scraping" Exchange's WebDAV interface for calendar data: Peter Krantz: Exporting Exchange calendars to Apple iCal over HTTP and WebDAV Ryan Hadley: Goodbye Outlook rexchange