I would not freeze buttermilk pie...you will end up with a weepy pie, probably. Just bake it fresh and enjoy it...use the balance of the buttermilk for rolls you can put in the fridge and pinch off to use when you want hot rolls, they keep over a week...use some in buttermilk dressing....I love to drink cold buttermilk with crackers...LOL...you can make buttermilk salad and freeze that and it is wonderful....
Never heard of buttermilk salad????
I make all our cornbread, biscuits, loaves of bread, some cakes and stuff with buttermilk. It keeps a long time. Except around here.
My Mom and Dad used to love cornbread crumbled up in buttermilk and a green onion on the side. I only use it for baking.