I have posted about this once before, but I will mention it again. I live in Tokyo and I can play the koto a little. I also know a good professional player (my teacher). If only I could find a suitable studio, a good sound engineer and enough money, I would probably try creating a koto library myself.
I have seen (in music stores here) some very inexpensive locally produced sample libraries of Japanese instruments, but they are not in any sampler format (i.e. they are just audio CDs) and tend to be more loop/phrase oriented rather than multisamples. I have not bought any of them so I cannot say for sure though.
I understand from the information on Ethnoworld vol. 2 (from Best Service/EastWest) that it does include a Chinese Zheng, which is an instrument very similar to the Japanese koto and would make a good substitute if you just need the sound.