Hi All,
I figured out a workaround for this issue in my office, but it's not nearly as elegant as I was hoping for.
I just gave up on finding a solution integrated with Zimbra and set up a Open Directory-style OpenLDAP server on another machine. Though they are intended for setting up Apple's Calendar Server, the instructions at
applecalendarserver [Me Admin!] explain quite well how to create an Open Directory mimic using vanilla OpenLDAP.
With that running, I just slapped together a php script which takes the Zimbra users via LDAP and reads them into the Open Directory-style LDAP. (It gets run by cron once a day.) I've attached the script to this post as a .txt file, but it would properly have .php as its extension.
I know there are much better ways of doing this, but given that it only affected auto-completion in one program, I didn't have the resources to get anything better going. I hope it's useful to someone though.
Cheers.