Are there any tags designed specifically for this purpose?
There are no tags specifically designed for transformers, as far as I know. The appropriate tag to use would depend on your requirements. For example, if you needed to interrogate a tag from a distance of 10 meters (33 feet), that would require a different type of tag than if you were reading it from 2 meters (6.6 feet) away. If you needed to affix the tag to the outside of the transformer permanently, you would require a model that could be protected against the elements. And if you needed to store information about the transformer, you might require a high-memory chip in the tag.
I would suggest you contact Metalcraft, Omni-ID, Syrma Technology, The Tag Factory and William Frick & Co., and explain to them what precisely you are looking for. One of these companies should offer a tag that meets your needs.
—Mark Roberti, Founder and Editor, RFID Journal