How can you keep a smoked pork loin warm after it is smoked for serving in a couple of hours later?

I think there are a couple of ways you can try to do this. First, brine the meat the day before cooking. The salt not only flavors the meat, it tenderizes it by breaking some of the protein strands, and IIRC, it also creates a "crust" which helps retain moisture inside the meat. I would also serve the meat with at least a jus, or a gravy, so its sitting in a moist environment. And keep the dish containing it covered, to keep as much moisture inside the dish as possible.