For the pulled pork: In a small bowl, mix together the cumin, coriander, salt and oregano. Rub the spice mixture all over the pork roast.
Heat the olive oil in a medium skillet over medium-high heat. Add the pork roast and brown on all sides, about 3 minutes per side.
Meanwhile, in a 3 1/2-quart slow cooker, combine the onion, garlic, chipotle and adobo sauce, beer, and orange zest. Add the browned roast along with any pan juices. Cover and cook on low for 8 hours or on high for 6 hours.
When the pork is cooked and fork tender, shred it using two forks. Add the shredded meat back to the slow cooker and toss with the onions and cooking juices. Keep warm in the slow cooker for people to help themselves and assemble the tacos with all the garnishes.