It's still tough dang it!
Do you have a circulator? Sounds like a job for sous vide.
Maybe just go with a simple brine.
Sous vide did come up in conversation, alas, no immersion circulator. Might be a reason.
Thanks for your help!
I you are going to fry the birds, I would skip all brines and marinates. A marinate will only add flavor to the skin of the product. The quail should be naturally tender. Save all of your sauces for after the quail is cooked. Down here sometimes water is tight and some of the birds are just tough. They are best stewed or braised.
It's not unusual for my wife's side of the family to drop of game birds for us. I always keep a bag of Aunt Jemima pancake mix in the chest freezer. I break that out, mix, dip and fry while they are still there. I li9ke sweet with fried birds. A very loose Asian peanutbutter dip goes well with it.