Cast iron - which rotors are typically made from - is a difficult material to plate. The cast iron microstructure includes free graphite, which provides a poor surface for plating adhesion.
Plus, good luck finding shops that can cadmium plate for you.... especially in California! Cadmium (like lead) is considered a heavy metal and the EPA is providing plenty of disincentive for shops to plate cadmium.
What about using high-temperature paint... similar to what's used on engine blocks?
95 E320 Cabriolet, 131K