Rather than trying to get rid of the heat (after all, burning a given amount of fuel will produce a given amount of heat, regardless), perhaps it would be worth considering putting a shield between the various exhaust components and the seat. Think of the material we all used in chemistry class in high school - your exhaust system can't be much hotter than a Bunsen burner. I've seen some material like this that was not just heat-resistant, but that would not break down due to moisture or other agents...