I really don't understand the fuss about the water level. Just tap into the house's piping, and run a small 1/4 line to a float valve. Common parts at any hardware store.
As for your setup, it's a dual loop (duh!). I assume that you would set the radiator at the lowest point possible, while allowing the air to vent through it. The question is: is it worth it?
The water would fall through the rad, and assuming that it doesn't restrict the airflow, it would drip down to the bottom (hot) where it would be pumped back up, to the shower head, then drip down (and cool) itself from the airflow.
But some of the airflow will include heat from the rad.
All in all, I believe that it would not be much of an improvement over a straight rad-and-fan combo, or a straight bong. If anything, you might try seperating the rad, and letting the flow go through the bong (an arrangement in series).
Since the bong requires a lot of pressure, it would have to have its own pump, but you were ready for that (right?).
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