Hi
is_in probably won’t do it. nominatim doesn’t pay attention to it, and many opinions consider it a redundant tag … is_in is implicitly defined by an object’s placement within other features.
http://wiki.openstreetmap.org/wiki/Key:is_in
I searched for other threads on borders and found this one, http://forum.openstreetmap.org/viewtopic.php?id=8475. Looks like there’s no conclusive way to handle this, and much consideration of the political ramifications.
I’m definitely aware that there are no easy solutions. I’m not here to create problems, but to freely discuss mapping, and particularly at the request of someone.
-Mikel