Th is is a complex answer. First of all, it is definitely possible. I've set it up on our production system. But it wasn't easy and we cannot integrate our CCM Directory with AD, athough we have no reason to want to.
The important thing to remember is that the Corperate Directory just requests a XML page using an ASP call. The default setup builds the XML Directory page dynamically from the DC Directory LDAP server, using a LDAP ASP tool that Cisco provides. This tool is limited in that it will only search on first name, last name, and phone number. In order to get it to work in a Mulit-tenant environment I had to find a more flexible LDAP utility that could be accessed from an ASP page. I found one at www.ldapservices.com, although it wasn't free (free trial, though).
With that utility, I rewrote the ASP pages, creating a seperate page for each tenant, which added a hard-coded search criteria of 'Department' to all LDAP search, thus limiting each tenant to only seeing entries in their 'Department'.
Now, it would be possible to redirect that LDAP search to a seperate AD server in order to get a directory. However, this would only generate a directory from AD, it does not create true AD intergration in terms of device association or CTI permissions. I don't see how you can integrate AD in a multi-tenant system unless you combine all tenants into a single AD structure.
