As I understand it, the chief tenderizing agent in buttermilk is not the minute amount of salt present, but the acid. That would make it a marinade, rather than a brine. Marinades soften tissue, brining retains moisture.
EDITED TO ADD:
I'm not trying to split hairs here. And, some on here are probably much more experienced cooks than I am, and undoubtedly those who process their own birds are completely out of my balliwick, but in my understanding of handling meats, it would make a great deal of sense to brine a tough bird before freezing, but not so much for marinating before freezing. Marinating should be done just before cooking. So, if they were my birds, I would brine then freeze. Then if I wanted to try marinating in buttermilk, I would do it with a thawed bird.