Iain,
If no one can suggest a Categorization method, perhaps one or more keywords
in the note portion of the record will allow you to group similar records in
a search. Is grouping appointments the reason for categorization?

Given that Outlook does not create reports of data it holds in the calendar
(I've been told) you might look at a tool created as an add-in (PowerToy) for
OneNote that opens OneNote for notes instead of the Outlook notetaker.
OneNote might be able to do the categorization for you.
Jonathan