Update ... I just read a post from Lindsay in the Mayflower ancestors user group. She had an idea for her problem and I think it's the exact right one.
She wrote: "What my point was for the G2G thread you included was that I wanted a useful way to pull up my veteran ancestors, immigrants, etc but dont know how to do that. Maybe a work around could be pulling up categories by MY ANCESTORS only ...".
This could, theoretically, work just like a surname index page. You
can see everyone with the surname, or you limit the display to just
the people in your Watchlist. So, you can go to the Stough index and
see all the Stoughs, or just see the Stoughs in your family.
As Lindsay says, this could work for categories. If categories are
used comprehensively, you could, for example, go to the Mayflower
Passengers category and see everyone who's a Mayflower passenger, or just see the Mayflower passengers in your family.
This might be able to roll up through the category hierarchy too, e.g.
so all veterans could appear together, even though some are Civil War veterans, World War 1 veterans, etc. But that would be tricky.