Create Zimbra Zimlet to Synch Cross-Domain Calendars

I am looking for the creation of a Panel Zimlet for Zimbra OSE 7 server which will allow a user to enter details of their Zimbra user accounts on other non-integrate Zimbra servers (cross-domain) to connect multiple default calendars across the domains with full multi-directional synch between all configured calendars. Based on my pre-work on this project, it will need to be completed by using the CalDAV functionality built into Zimbra. To help clarify the project, here is a real scenario which will be solved by the resulting Zimlet from this project: A user (let's call him Nathanial) has three Zimbra accounts on three separate Zimbra servers that belong to three separate companies. Nathanial needs to have a single, unified calendar between all three so that his busy/available times will display properly to anyone attempting to schedule a meeting with him at any of the three companies. So whenever Nathanial creates a meeting or accepts an invitation, all three of his DEFAULT calendars are updated with the new calendar entry. It cannot be a "new" calendar made for syncing, it must synch the standard/default calendar of each user account. The Zimlet should only need to be installed on the "hub" Zimbra server and have a setting for how often syncing should be performed. Please ask clarifying questions and I will gladly post more information to the project.