Put pork and dry ingredients in a re-sealable bag, seal the bag, and shake to evenly coat the cubes. Refrigerate for at least 15 minutes.
Preheat a medium non-stick saucepan over medium-high heat until it is hot enough for a squirt of water to sizzle on it. Add the oil. Spread out the pork cubes into the pan. Cook, rotating as needed, for about 5 minutes, or until the pork is browned on all sides.
Reduce the heat to medium and add the orange juice, vinegar, and smoke flavoring. When the mixture comes to a boil. Reduce the heat to low so the mixture simmers. Cover the pan.
Cook, stirring occasionally, for 1 hour, or until the pork is very tender. With a wooden spoon, shred the pork pieces and mix in the barbecue sauce.
If the onion is desired, 5 minutes before serving, coat a small nonstick skillet with olive oil spray. Set over medium heat, add the onion. Cook, stirring occasionally, for about 5 minutes, or until tender.
Meanwhile, lay the bun halves, cut sides down, in a nonstick medium skillet set over medium heat. Cook for 3 to 5 minutes, or until toasted.
Put the buns on serving plates. Serve half of the pork mixture onto each bun bottom. Top with onion and more barbecue sauce if desired. Cover with bun tops, serve immediately.