Why would they need to let their hair down, it seems to me if someone believed this than anytime they put their hair up, or tied it in a ponytail, or otherwise restrained the hair, it would it would be a sin to them.
Everytime I hear about this it reminds me of that witch with the green face on Bugs Bunny cartoons, every time she flew away on her broom all these hair pins would fly around.
ARPH