I tend to agree with the above. Like he said, the 2nd gen Prius uses mass airflow rather than speed-density. I also doubt ramming air into the intake will do anything for fuel economy. It could potentially help with power at high speed / full throttle. But if you're trying to hypermile at WOT, you're doing something wrong.
Personally, I would not want to modify the drivetrain in the Prius because of how it's configured. There is a balance of power, so to speak, between the gas engine and the electric motor. If you upset that balance with engine mods, it could have unforeseen consequences.
Actually, that could explain why you lost fuel economy with a hot air intake. The reduced power of the gas engine probably reduced the overall efficiency of the hybrid system.
Anyway, it seems like your main goal is to find somewhere to send the hot air exiting the radiator. How about a hood vent? I'm not talking about a louvered race car hood, but rather a smallish vent placed strategically in a low-pressure zone. IIRC the best place to put a vent is right behind that sharp curve at the front of the hood.