Update 2:
Was able to find a used hardness gauge for $160 to test both the virgin rim and the powder coated rim. One of my rims will end up with a 1/2 inch wide semicircle indentation so plan to test the barrel portion.
Will finally be able to tell if the heat cure softens the rim(most likely not) or hardens the rim further making it more brittle(possibly) and to what degree.
It has been a maze to try and figure an answer to a question that no one wants to answer and indeed keep clouded in legalese.
Rim makers say don't do it as it will change the structural integrity and they have there modification disclaimers to void the warranty.
Powdercoat makers say it should be ok, but also have disclaimers warning that a person should take in all the facts and concerns before making a decision and to speak with powder-coater.
However the facts and data are not readily available and the powdercoat makers whether it's PPG, IFS Prismatic, Sherwin Williams, etc still make and supply powder coat that need to cure at 400° F.
And the actual small powder-coaters say they do hundreds of rims and no one has problems.
And the question is:
Will the heat cure at 400° F cause the rim to change so substantially that the wheel becomes so brittle that it could potentially lead to loss of life and limb.
Based on the amount of powder coat rims that are actively on the road, no first hand reports with pictures (that I can find) of rim catastrophic failures, imo, am leaning toward the conclusion that the rims are weakened and may become slightly more brittle but not enough to be detrimental to the operation of the vehicle.