I'ver been over there and paddled with siam rivers. Jason, the guy that runs it, is an awesome resource and would definitely help you out. Just shoot him an e-mail and he'll give you some info. He's also got a great place to stay in Chiang Mai. If you have any questions you can e-mail me [mail address protected from spambots with javascript] . Eric Southwick is also in Bangkok and he's got good beta as well.
peace
David