It's not an easy row to hoe, as you're going to need to make your server handle all the SOAP requests and responses that the Zimbra server does. You can check out
soap.txt in our subversion repository to get a feel for the magnitude of the project.
Your server will also need to be able to parse XML requests and send back JSON responses, as that's what the Zimbra client currently expects.
And you should note that
you may not rebrand the AJAX client. The "Zimbra" logo stays where it is and cannot be hidden in any way. That's just the terms of the license.