Im not sure what the labyrinth section is but I paddled a class III section of the naranjo last november on a guided trip with Mad Max Young from Denver. It was an awesome class III, some really nice rapids and spectacular scenery. Sounds like you're talking about a more difficult section of the naranjo though? The rio savegre also near quepos was a fun larger volume class IIIish run, if you do this one watch out for the very large crocodile who makes his home at the take out though.
Im curious, are you going to rent a car for shuttles or hire a driver/van? Renting boats or bringing your own?