First off, I don't know anything about the Koto. So I don't know how well the concept would work. But, since you claim that you don't know how to compose for the Koto as well, I think it would be wise to so some indepth study and research.
http://www.kotonokoto.org/about.html
I will say the same about the Harp as well. Do some research on the instrument. There are some issues and challenges with the harp that you will need to always keep in mind specifically when working with various keys for the harp and the pedals. Besides you will want to know how to get the best out of both instruments (on your composition level) is you are composing for them.
Harp Spectrum
I like the theme of this movement and I think you could do more as far as variation the theme and development as well as thinking about the form. But, I feel that you should take care of your Koto and Harp understanding first.
