How to disallow the ASCII input

An anonymous forum poster raised a question on how to allow users enter only Katakana characters into the Akn controls. If there is anybody with such an experience it would be great if you could give the guy a couple of hints. Unfortunately I never had to work with the regional specifics and don't know much in the area.

Some source code is included into the forum post

P.S.
On a side note: XP2006 is great!