Well, the style is in naruto D20 to give some cool stuff to katana using shinobi :P. But in essence the style could be passed on Mibu family as the family kinda comes all over the place from any ranks and levels of the society so a master swordsman who had mastered Hiten Mitsurugi-ryū and it's so passed only in the family (but practioners could found also elsewhere some corners of the world).