eb,
I'm not sure what you can use it for but someone uses it -- DH used to work for the AZ state dairy cooperative and they sold it by the ton.
They made cheeses, butter and dry milk but they dealt only with the state co-op's "excess" liquid milk, that left over after most of it was sold for drinking or made into yogurt, ice cream, cottage cheese, etc. They bagged up the whey produced in their cheese/butter/dry milk process in 100 lb bags and sold it by the semi-truck and railcar load in the US and abroad. Who knows what it ended up in but I'm guessing it would raise the protein level as you can purchase "health shake" products from TJs that are whey based.
I have used it in my dog food the few times I drained yogurt
Good question, maybe we'll figure something out in this thread....