The Hard Coat anodize quality depends a little on what the welding filler was used. It also depends on who is doing the anodize. That is why we ship the radiator 1000 miles to us to anodize them.
Heat is not a big problem, we have been selling anodize front drives for years. Lots of hard coat parts are sold like Earls fittings, they do not change color. Remember the radiator is only at 200 degrees which is not real hot. Sunlight (UV) is death to any anodize.
Powder coat is not a option if you want the fins black, will absolutely ruin the cooling efficiency. You would have to paint the fins with a special paint for radiator fins, tape them off anf powder caot the rest. Might as well paint it at that point.
Here is Chris Radiator. The welds are SLIGHTLY different color. a LITTLE browner then the rest of it. But gives it that anodize look.
