Search was a challenge because the video that used it played it at half-speed, but after 8+ (!) years I finally discovered the song. In fact it wasn't from a Miyazaki film but was written by an obscure Japanese musician, presumably a royalty-free one:
SAMURAI SPIRITS - Hakone (https://www.youtube.com/watch?v=u_70LeOlRWA).
Finally solved, but can't find a way to close my own post.
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