I hired a guide out of Sapporo this past August. The link is here: http://www13.plala.or.jp/somerise/.
We threw #10 Stimulators, #12 EHCs, and some traditional wets on steelhead hooks. Despite the seemingly-huge flies, the fish lived up to their diminutive reputations–countless 4-7″ yamames, a 10″ iwana, and a couple smallish browns.
That said, the guy’s latest report (9/1) claims that the salmon and sakura masu are entering the rivers in record numbers this year.
Last, keep an eye out for bears. Folks up there are seriously concerned and warning signs are posted at every river. Bells and dogs seemed to be standard issue.
Best of luck and don’t forget to hit up a shop to stock up on tying materials. There are so many cool Japanese hooks, tools, and materials that never make their way over here.