This is just an idea for consideration. I run a one-name study for Coggrave (a rare Yorkshire surname). Would it be possible to create a sticker to use on profiles of people who don't have the surname, but who have a Coggrave ancestor?
(it could say something like "Josephine Bloggs had Coggrave ancestors.")
A category doesn't have the same visual impact; a sticker might help people who have Coggrave ancestors to find out more about the project and consider contributing.