Can you explain the use case a bit further?
As best I can tell, you're talking about the ability to set permissions on individual calendar items ("cal_item" object type), separately from the calendar itself.
The data model certainly permits this, though one would have to turn off permissions inheritance on the individual calendar item as well.
If this is in fact the use case, I'd be interested to hear more about situations where this would be desired behavior vs. creating a separate group or perhaps a subgroup.
