I imagine that one heats better than a PVC one. Buying the black ABS pipe would help, but plastic is still a good insulator. It does seem like an aluminum tube on the roof rack would cool a lot while driving, though. There are some great Australian systems that use a water-water heat exchanger with the engine coolant and an electric pump. You can park next to a river or bring up a bucket of water. Again, you can replicate these, since home brewing has brought the cost of heat exchangers down, but it still won't be like hillbilly cheap. The best one I saw used air-type quick connect fittings on the vehicle and a temperature regulating valve. The problem is that the temp is fine as long as water keeps flowing, but when you stop the water mid shower (army style) the water gets super hot for a couple seconds.
The super cheap method I've used is to fill a $12 solar shower bag as soon as getting to camp and putting it on the intake manifold. After dinner, the lake/river water is at least up to ambient. Then you put the bag on the roof rack and let gravity deliver the water. Of course you can also heat water on your camp stove or campfire and add hot to the bag.
It's best to fill your water up and carry it all day since that gets it to room temp, but of course you need a water carrier that you trust not to leak, or just use a rear mount jerry can or something.