I don't have suggestions
for an interface, but why not just start with a SpecialPage and see what
happens? Once the functionality is there, suggestions for how to
better use
it will come out of the woodwork.
Several of us at Wikipedia did some work on an interface for
incorporating category intersections a while back. It can be found at
shortcut [[WP:CI]] or full url:
http://en.wikipedia.org/wiki/Wikipedia:Category_intersection
--Samuel Wantman
[[en:User:Sam]]