When you talk of firing are you talking about putting it in the oven? or a kiln? Polymer clay goes into your regular oven. The metal nametag, might not stick to well, but after you do the piece the way you want it, with the metal in it, you could always use E-6000 glue, that will hold it in place.

earthenware and polymer clay are completely different things. Polymer clay isn't fired, it's just baked at low temperature in a regular oven. It's not actually clay either, it's made of resins and plasticizers Polymer. clay will do fine with the metal. As ladonna stated it may not stick to the clay, but can easily be glued after baking. I think this sounds like a lovely way to commemorate your kitty.