Its not Open Content
While developing my commercial oriental setting - Kaidan: a Japanese Ghost Story setting - for Pathfinder, I contacted Jim Buhlman and he verified that Oriental Adventures is not Open Content, the same is true for Rokugan D20, so copyright prevents that from occurring. That's why I've been developing a distinctly Japanese based setting from the ground up.
Besides, no such thing as a Wu Jen, not really - I've gone the route of Onmyoji Wizards and Jugondo Sorcerers for Japan based arcane spell casters, Wu Jen is closer to a chinese idea, though completely fictional. I really wish Gary Gygax would have introduced Onmyoji as a spellcaster, and not invented Wu Jen just to get a spellcaster into his setting.
GP