If I know I am going to freeze, I'll foil the butts when they hit about 170. You'll get a lot of drippings in the foil. When pulling mix the juices in with the meat. Should provide plenty of moisture when reheating.
I usually keep apple juice with a bit of apple cider vinegar in it to mix in the meat just in case the meat has to spend a long time in a roaster / crock pot, etc.