Actually, if you simply moved the radiator below the cold air intake for the bong, I think you'll be set: The hot air will be more or less limited to live between the rad and the bong's water level, and the bong's air intake will stay cold and simply interact with the bong water. This will also mean you get better airflow, since the air does not need to force it's way through the rad, and the rad will get a better interface with the water b/c the air won't be trying to push it the wrong direction.
Code:
.:H:. Water hose and steam :)
+-M-+ Showerhead
|:.:| ___ Fan blowing in
|.:.|/ /
|:.: /
|.:.|
[===]=== HeaterCore
|~~~| Water (duh)
+---+