This is a neat idea, but...
So one can write &hiragana_wa; or &katakana_chi; This isn't likely to conflict with anything.
Kana Unicode table (in "English") is on http://pl.wikipedia.com/wiki.cgi?Kana
In general, why would it ever be common that one would wish to put Kana into non-Japanese wikipedias?
At least at first thought (and I'm totally open to have my mind changed!) the only place in an English or Polish wikipedia where showing Kana would make sense would be an article _about_ Kana.
--Jimbo