Steel may not be the only magnetic metal but the lightest most comparable to the wieght of binding would be aluminum, and thats not magnetic.
Remember in order to have a proper magnetic grip, the metal your griping HAS to be a certain thickness no mater how powerfull tbe magnet, example look at the giant scrapyard magnet u guys are using as a comparison, notice all the thinner metal just falls off. Any metal you use will be heavier then the wieght of plastic and foam bindings so yes youll add about 3 times the weight and the shock absorbtion would then be all in your boots and centralized to werever the magnet and metal meet making a localized pressure point.
So weight saving is outa the question.
Life is a giant high five, huck it every chance you get.