I think the FS Tented Camp should be able to pre-arrange the Mekong boat from Chiang Rai to Louangphrabang. Otherwise you can do it yourselves on the spot. There are no big operators. It is like a (shared) 6-seater water taxi, except that I recommend to hire the whole speedboat for yourself, so you can define the travel speed. The trip takes at least 6 hours, at a cost of ca. US$ 200 for the whole boat. It is basic, adventurous, a bit tough but beautiful.

There is also a slow but somewhat more luxurios version of the Mekong boat, overnighting in a jungle lodge, but I don't think that the time / luxury equation is what you would favour: http://www.symbiosis-travel.com/laos...-river-cruise/

I just saw on Wikipedia that Lao Airlines now operate a flight from Chiang Mai to Louangphrabang. That may be an alternative that I didn't know about so far.

Flying via Bangkok would be more strenuous than taking a boat IMO.

Here is some general info: http://www.travelfish.org/feature/119