Doesn't copper and steel create a galvanic reaction, like a current between the two. I think the choice of valve oil, and how to store without freezing the copper valves to the casing.
Anyway an interesting thought, copper is so soft, you'd need a way to replate them often - so maybe no viable.
BTW glorybe. Congrats, the oldest Zombie thread raised this year. 3 years was the previous, 4 years is a good effort.