For those looking for it, the guitar tab is available on this topic in the forums on Ally Kerr's official website in PDF format. Though this is for a single guitar; what you hear during the intro of Mushishi is actually two guitars.

While folding clothes, I decided to just have something play in the background, so I stuck in the first DVD of The Twelve Kingdoms (A series I hold dear and have watched many, many times). I finished before the first episode was finished, so I tucked into bed and watched the rest of it. As the ending credits came on, I hit on a rare moment of inspiration, got up, grabbed my dusty acoustic guitar, and started playing. Well, started guessing at least. The chord progression to the ending theme, GETSUMEI FUUEI (Song of the Moonlight) sounded too familiar to me.