I thought that Zimbra was an OpenSource project?
You are welcome to use, modify, and redistribute ZCS Open Source Edition free of charge, provided you do so according to the terms of the appropriate License.
Zimbra Collaboration Suite (ZCS) source code is licensed under the terms of the Yahoo! Public License (YPL).
How about just "copying" the zimbras-ldap module to make one for us?
Even Joomla project has finally managed to "support" ldap authentication. (v1.5)
Joomla has been notoriously bad with authentication because their CMS comes from Publishing sector that has never cared about such things.
We could just go that way. Not really adding anything in ISPConfig execpt the "support" for external ldap queries.
I don't see any way around this. The longer we wait with this the more it will hurt us when the circumstances force us to support ldap.