I'm not sure why you're doing anything special with LDAP for this.
Not all features of iCal delegation are supported by Zimbra. I believe that you can't create a delegation through iCal. What you can do is mount a shared Calendar in ZWC, set your Calendar preference (in ZWC) to "Enable delegation for Apple iCal CalDAV client", and then you'll be able to use the delegation tab in iCal preferences to show or not the calendar.
If you don't use "Enable delegation for Apple iCal CalDAV client", then when you mount a calendar in ZWC, it shows up as "just another calendar" in iCal. Still functional, but not quite as elegant.
See also
Bug 45853 – RFE: use ical delegation tab to share Calendar