I haven't been in this situation so this all wild guesses.
I've seen something about freezing tobacco only once: the cigar company will slowly dry their cigars, freeze them, ship them overseas, and thaw them slowly and bring them back into case slowly to sell them.
My guess is that, given our chaotic winters, it may occur that you get a warm day or two, the temperature rises in your barn attic, along with the humidity level, which will get some water in your leaves, then the following day the temperature drops by 30 degrees and the leaves freeze suddenly.
I don't have any knowledge about what effect this could have on the leaves, but I know what happens to lettuce in the freezer so, personally, I would try to avoid having my leaves freeze if they're not dry.
That being said, if you know the barn attic never freezes, it's another story.