[Christ]

Cut a hole in the hood, insulate the engine bay (belly pan, etc.) and put plexi in the hole. Park in the sun.

Thermosyphon probably won't do much for you here... it takes a long time for the effect to become... well, effective. Something as simple as a fish tank pump or even smaller would be sufficient. A condenser from an old A/C system would make a nice heat exchanger, or even the tube thing from the back of a fridge. Either could be mounted "cleanly" to the hood, and covered with a thin skin that would only cause minor aero problems, and be uncovered when necessary with a few screws or whatever.

If you really wanted to get fancy, you could cut a hole in the hood the same size as the heat exchanger, then fashion a bucket-thing for it to sit flush in, and mount it that way, using a flat cover when you're driving.

If you park near the dryer vent of your house, and your underhood area is insulated well enough, you could duct the dryer vent under your hood when your wife or you are doing laundry.