Really good question, any one have any ideas??
If the group is not materialized, I would imagine the case has to be better. Could you instead do a join with the category placings, then extract that?
I don't believe ad hoc groups are materialized in extracts, everything I've ever read or heard says that once they get past a certain size they go into a temp table for data sources that support them.
I know that at one time with Tableau v8 when I tried to create a really massive CASE statement that had several hundred members, I ran into a character limit, so I'm not even sure a CASE statement that big is possible. I think you're better off following Alex's advice to create a separate table to hold the associations, then you can left-join that in a multiple tables connection or custom query, or even use a Tableau data blend.