"But our modern (and unwavering) image of Santa Claus was cemented in the popular imagination by that ad campaign for Coke."

I disagree. I have statuettes of Santa in a variety of outfits. Dwelling as he does at the North Pole, Santa can look south to every culture in the world, and he has outfits from many of them. Naturally the various forms have to be adapted for survival in the rigorous Northpolar climate.