Brian hit the reason why buttermilk, alone, does not replace a brine. Salt is essential to how a brine works. My only concern about using buttermilk in the smoker is that it is a dairy product. When you fry or grill chicken, the buttermilk in the batter goes from cold to cooked pretty darn fast. In the smoker, you're going to have a dairy product sitting at an unsafe temp for quite awhile. Maybe someone else has thoughts on this - I'm just kind of thinking out loud. Not sure if it would be a problem, but seems it could be a great opportunity to grow some bacteria. Something to think about.