DNBK and Koryu?
I would not assume that there is any link with the modern DNBK and with any Koryu groups currently practising in Japan.
What I have seen of the organisation of the Butokukai is that it seems to promote various groups of gendai Budo forms but no Koryu.
There seem to be links with Judo, Karate-Do, Kendo/Iaido, and possibly some other modern forms (Like the British Jujitsu group you highlighted earlier) but there are no links named with any of the Koryu. (This, as I stated earlier is the role of the Kobudo Shinkokai and the Kobudo Kyokai groups mainly...).
As far as legality goes I am sure it is legal....That is not really the point that Mark and others are making..You seem convinced that the group maintains a 'overseer' role in Japanese Budo, both Koryu and Gendai, and that the DNBK is a 'major' player in Japan. This, as has been stated already is not reality..It seems a shadow of its former self and thank God that it is...The role it used to play is one of the things all countries should be trying hard to eliminate. (IMHO).
As far as the email address goes I can't help...If they don't have a website it may be harder than normal to find one, more personal certainly..For your question though the two Koryu groups I mentioned would seem to me to be the right direction for inquiries...