Has anyone experimented with "per-user categories" as an extension? The commercial wiki Confluence has this feature: any category (called a "label") that begins with "my:" is local to the current user. So people can tag pages with their own categories for their own reference, and these categories are not visible to other users.
So if you added to an article:
[[category:my:important]]
this creates a category "my:important" that's visible only to the logged-in user who created it. So people can create their own locally organized sets of articles.
DanB