My bother showed me a trick on a pork loin he smoked this weekend...not tenderloin but the big ol' 1.99/lb loin that it tough and dry and lean. He seasoned it with s/p and then coated it with Mayonnaise. Yup, MAYO
About a 1" thick layer.
Meat turned out moist, tender and absolutely wonderful. Never woulda believed it but it works for smoking.