Although I like the Zimbra Web Client for some features, I'm still more comfortable using Kontact for email and calendaring. So I've been using Kontact via IMAP to our Zimbra server and that seems to work fine. I've got an LDAP connection to access the GAL that is also working nicely. In my IMAP folder list I can see Contacts, and can open them in kmail to list them out as "messages".
Kmail has a setting, Misc/Groupware, that allows you to "Enable IMAP resource functionality", and it appears to want to store contacts and appointments in IMAP folders. I checked it, and specified the Zimbra set of folders, but it errors trying to create a Calendar folder. It presumably has already been created by Zimbra, but kmail can't see it. Can the Calendar folder be set to show through IMAP? If so, would it be writeable and useable from kontact?
I went ahead and created an address book as "Addressbook on IMAP Server via KMail" and it almost sort of works. It will show a few of my contacts that I clicked on in the Contacts IMAP folder, but it doesn't show the whole list. It might be half baked because of the Calendar error though. It also appears to be read only.
If I could access contacts and appointments through IMAP Kontact would be much more useable for me, and no need for any kind of plugin. As it is, I keep the web client open to manage my calendar and use kontact mostly for email.