There is more than one Yairi - it's a bit like "Smith"!

If your neck joint cannot be split for a neck set, remove fingerboard, and put a tapered blank, tapering from zero thickness at the nut end to whatever is needed at the other, onto the neck, shape it, then replace fingerboard with or without refret as necessary. Don't try this at home, but it can be very successful in the hands of of a skilled luthier, who can at the same time and in the same way correct excessive neck progression. I have a friend whose 12-fret 12-string was revived this way, and at the same time it was changed from a tailpiece guitar to being a pegbridge - all by Graham Noden in Denmark Street, London, for surprisingly little money, and it sounds magnificent, even though the neck is still enormously enormously enormous.