I have smoked a 4lb pork loin roast. It came out real tender with lots of flavor. I let it sit overnight in the fridge covered in rub and put it in the smoker for a couple of hours. I pulled it when the internal temp was at 140. This was on the lower rack, with some ribs on the top rack. I let it rest for a bit before slicing. My thinking is that if it was still pink in the middle, I'd just grill off the slices to finish. I was surprised to see that it waw pretty much cooked all the way through, not dried out at all. I sliced the roast into fairly thick "chops" and grilled them anyways to get some nice grill makes on them.
To do it again, I'd probably cut the roast in half, lengthwise to get the rub inside, then close it up and tie up with some butchers twine before letting it sit overnight.