The only time I did dynamic hair with a hat (see "Nice Hat" in gallery), I just made the model bald under the hat but loaded up the hair just above the brim. I think I also once heard a suggestion of doing the hair on the inside of the hat, just above the brim, growing in towards the head. Do collision against the hair and the head (or skullcap) and then lower the hat onto the head during the animation. I've never tried it, but it sounded intriguing.