Osgood,
Your link was bad. I think this is what you meant
http://www.ecoscapetours.com/web_files/sunset.html. That actually sounds like it could be a very interesting tour that I had never heard of before. This is one of the outfits whose main offering is an All-in-1 tour (and happen's to be the one I went on). This tour is a variation of that that might appeal more to some of the guys around here since it allows you to sleep late (or go for a morning MP session). It leaves SJ between 1-2PM and arrives back around 9:30PM (the downside but still not too late for squeezing in some evening entertainment). It includes a drive through Braulio Carillo Park, a Sarapiqui River boat trip, a guided nature walk at sunset and dinner at a riverside jungle lodge.
A night cruise on the Rio Tarcoles near Jaco might be somewhat mosquito infested, but I'd be more concerned about being out in a small boat at night surrounded by a bunch of crocodiles. Mosquitos can be a problem in other areas too, but a good repellant can work wonders for that (see my recent thread
http://www.costaricaticas.com/phpBB2/viewtopic.php?p=58056. There are actually a few places in CR where one can go on a night jungle tour. Monteverde is another place that comes to mind. There is a surprising amount to see and hear as many creatures only come out after dark.