If you make steamed dumplings with your cabbage, you can use the outer leaves to line the steamer trays. Helps keep the dumplings from sticking to the bamboo (or whatever material you steamer is made of).
You could also use them to line the pan if you make galumpkis (stuffed cabbage rolls). My mom-in-law's recipe says to line the pan with the outer leaves of (regular) cabbage, put the rolls inside, then cover them with another layer of outer leaves - keeps the cabbage rolls themselves from burning. I don't see why you couldn't use Napa cabbage leaves for this; i bet you could freeze them now & save for a later galumpki-making fest.