Of course, clothes can be hung outside for the warmth of the sun, but this seems way cooler.
As you're driving around your engine is already trying to vent heat to the outside, so you're mostly getting the heat "for free". The losses due to extending the coolant loop etc. will be comparatively minimal.
The only tricky bit will be carefully managing thermostats to ensure that radiator won't be melting something is the bathroom, your coolant will get hot enough to boil water.
A "it ain't dumb if it works" variant on that is to literally bolt a box to the top of your hood (depending on visibility etc.), then remove your hood's insulation, and possibly drill a hole to exchange air between the two. Anything in the "hot box" should dry as you drive around.
It should not, unless you are at a high altitude or your engine is faulty. Thermostats on engines target around 90c, but it does vary.