That depends on if the storage flaw is in the metal or from the person. Do we know if metalminds have to be the exact alloy like allomantic metals do?
Though the more I think about it the more Vin's earing becomes a bad example, I feel like sensing another's magical abilities is more like pushing or pulling on emotions, it is less measurable than something such as strength or senses. Though this is just my interpretation. Do we know what effects a steal spike would have on a pewter arm? If so it might be easier to figure out the correlation in feruchemy.
Though maybe I was looking in the wrong direction, perhaps the spikes we should be using are Iron, Tin, Zinc, and Copper. Those steal actual physical atributes rather than allomantic or feruchemical powers. This way the feruchemist could physically have more to store.