I am trying to do a similar thing by forwarding all zimbra mail to a mailarchiva system for obvious reasons. However, I tried what you attempted, that is, adding a transport map file to the postfix in zimbra, but I could not get zimbra to see the settings in the transport map file I created, and added to the config.
I suspect the problem has something to do with how zimbra uses the ldap file for the transport map, and not a standard hash transport file.
Would you please let me know if you got this to work, and what you ended up doing to config your zimbra if you did.
I would really appreciate the help.