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The problem with boiling the fuel is that most of it will then escape out the vent before it can re-condense. I'm sure this will more than offset any fuel gains from drag-free cooling. Even moderately raising the temp (and vapor pressure) will probably cause excessive evaporative loss of the fuel. We have to face the fact the fuel is not an acceptable coolant for this application. That's OK, there is still Evans or water and the rest of the wing surface to be used.
Look up "heat pipe" using Google.
The returned vapor will quickly condense in the cool tank. The only way it will not condense is if the tank, and it's entire contents, reach the boiling point of the fuel.
Until the entire tank and the fuel in it warm up to the boiling point, all of the vapor will condense on the walls and on the surface of the fuel in the tank. As long as most of the vapors are condensing, the fuel properties will not change.
This is why you need to monitor the fuel tank temperature if you are planning to use the fuel as coolant. If you dump too much heat into the tank, it will become warm enough to vaporize the fuel. You probably would not want the tank to become much hotter than, say, 140 F, I would guess.
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