The Sumatra application that takes your intermediate format database and converts it to ICS files can now also automatically post to Zimbra (saving you a crucial step).
The format is the same as you are used to:
zinsert -fo -tf_utc_to_timezone -cs DSN=Zimbra -post http://YOURSERVER/home/[login]/Calendar?fmt=ics -loginpwdb
will auto insert, reading the User ID and Passwords from the Users table of the database.
In the example above you see that user "Puffy Amiumi" gave the error "No TZID for TimeZone" because we had not yet mapped the Japanese OCS Time Zone to Zimbra format.