It's only implemented for Iberia at the moment, but the mechanic itself is region-agnostic so there's definitely the possibility of it being used elsewhere in the future.
We're not currently planning on expanding the map to include Japan, but we never rule anything out entirely. A lot can happen over the course the game's lifespan, after all!
In the meantime, there are several (quite lovely) total conversions that change the map and mechanics to medieval Japan.
devtrackers.gg
