So I have been asked to make a nightmarish beast of a calendar assembled across from various branch offices; with each office feeding back into one large (and probably incredibly slow to refresh) calendar.

The request:
Allow each branch office located all around the region to have their own specific and locally maintained calendar of events (closure dates, training opportunities, etc...). Each of those "branch calendars" would then automatically feed into a single "master calendar" that can show events from across ALL of the branch stations. The trick is that the master calendar would be accessible over the web for the public to view.

The idea is that a designated calendar person at each branch office could post their local info and avoid someone at the main corp office have to re-enter that very same data into a corp wide master calendar showing all the branch offices' events ON A SINGLE calendar page (that is what they are really wanting out of this).

If it was just a normal Zimbra user, I know this can be done by using shared calendars and just checking/unchecking the ones you want to see, but since a merged master calendar will need to be shared out over the web I was not sure how it could be done.

Any suggestions on a good way to go about this? There are about 100 branch offices, so I am trying to gather info before jumping in with both feet. Thank you for your suggestions.