If you're concerned about the meat drying out, or whether it will have enough moisture to steam the flat after you wrap, you could spritz the meat with apple cider vinegar or similar before you wrap. But really, you won't need to add much moisture to it in order for the bp to do it's magic. I agree that there won't be an airtight seal, but one is not needed for bp. It will retain a surprising amount of moisture, regardless.