As windfall, and I, pointed out you HAVE to have some air exchange for the bees to breath, so it is not a closed system. Second there is a certain amount of flywheel effect where it both absorbs and under other conditions gives off moisture. But as I said, my main concern is bringing the hive up to weight. I have no real expectations of any significant amount of absorption.
If you look at those little silca gel packs they add to everything from beef jerky to guitars, in theory they can't absorb that much moisture, but again, I think they act more as a flywheel, stabilizing the humidity, more than changing it and again, preventing condensation in the case of guitars or vitamins or computer monitors.