From your response to Andre, I think you might want to review the method that busy time information is both published/request and also access/replied.
What I mean is that the RFC 2446/iTIP gives the "verbs" around the "nouns" defined in iCalendar. It provides two mechanisms for exchange of busy time information.
Method 1 - Publish/Access
You periodically take a snapshot of your opaque VEVENTs on your calendar and publish an iCalendar object containing a VFREEBUSY calendar component containing your busy time information, for say two weeks. Folks with access to your "busy time URL" can GET that object using HTTP or some other file access protocol. Access to the busy time information is not too restricted, I think you would agree.
Method 2 - REQUEST/REPLY
Using the VFREEBUSY based REQUEST and REPLY methods defined in RFC 2446, you can request, via email busy time for a given time interval from someone. Of course, that individual can just ignore your request if they don't think you are authorized to get that information. Or if you are authorized, they can send a REPLY method with the properly formatted busy time information. This approach provides more secure, but asynchronous mechanism for getting busy time.
Hope this helps. More feedback on your original note later.
-- Frank