ask Loki at Rafiki to take you on the SuperLekker... it's a horse assisted hike way up to the headwaters, so you paddle a section that's only been run a few times
we did it at Easter last year and it's truly an amazing trip, only for those with a sense of adventure (3 hour hike, 2 hour paddle)
also you might want to check our new web forum
Costa Rica Creeking - Whitewater Forum
for more beta and possible partners
the deal with the rafting is probably a good one... I own a small rafting company in CR and can tell you allowing a kayaker to tag along is a risky proposition, so the fact that Miguel is allowing it is a good deal in and of itself, check with them well in advance and make sure the logistics are worked out for you