After some humming and hawing as to whether or not I should simply create my own pseudo-Japanese nation from scratch or else borrow one of the maps for Kazakura from the Oriental Adventures box set, I've finally settled on a third option. Frankly, I'm just going to use a map of Japan complete with original province names and the names of major cities as my starting point. Runequest Ninja (a very fine supplement by the way if you can find it) provides an excellent map of the Provinces as they stood in the (I think) Tokugawa period.
This has the added advantage of allowing me to incorporate various Japanese geographical myths into the campaign world -for example, the legend of a pit leading directly to the Buddhist Hell in Izumi province makes Izumi the perfect stand in for one of the major adventure locations in Masks of Nyarlathotep.
However, rather than simply call the country founded on these islands Niwon or Nihon and use an "alternate earth Japan", I'm going to incoporate them into my home-brew campaign world, Edarnia and call them "Hisui no Shima", the Jade Islands. Fortunately I've already established that Edarnia has an Underworld as well as dark, utterly alien Cthuluesque Gods even older than the Primordials who created Edarnia. On top of that, the game world also has as well as a malevolent force of non-life known as the "Corruption (amoung other names)," an excellent stand-in for the Taint from L5R. However, I like the sound of "taint" so it looks like the corruption has acquired another cultural-cetnric name. I'll be tracking (and using) taint points in the same manner as Insanity points are used in Call of Cthulu, with the added bonus that an increasing taint score also results in increasing physical mutation. Hmm, echoes of Ravenloft here as well perhaps?
Ezo, the northern-most of the Japanese Islands, was not particularly settled until the 15th century. Except by wako (pirates) renegades and Ainu (the indigenous inhabitants of Japan). Ezo seems to be an ideal stand-in for the Shadowlands of L5R , the Chaos Wastes of the Old World, the Blight of the Wheel of Time and about a dozen other settings combined. So that' what I'll use it for. The interior of the island will be a blighted wasteland of volcanic cloud, ash and a gateway to the Hell of the Hundred Million Kami. The coastal areas will be habitable, and home to a few hardy samurai clans and their retainers as well as a great many bands of ronin, wako and other renegades.