In a large bowl, mix together the chipotle chili, oregano, smoked paprika, cumin, coriander, nutmeg, sugar and salt. Cut the pork into 4 or 5 large chunks, and cover with the spices, cover and chill for at least 2 hours, preferably overnight.
Add the pork and spices to the slow cooker, then the orange juice, cider vinegar and the juices from the pineapple. Set on high, and cook for 5-6 hours, until tender enough that it breaks apart with a spoon. Pull it apart, turn down the heat to low, and cover until ready to serve.
Cut the pineapple into small chunks. Mix in a small bowl with the tomatoes, onion, coriander, lime juice, olive oil and chili. Season well, then set aside for 30 minutes to develop. Cut the avocado in half, remove the stone, scoop out the flesh and then mash to a coarse paste. Season well, adding a little chili powder if you like.
Heat the tacos as instructed on the packet, then serve up everything so that people can help themselves. Great with a small salad and fresh sweetcorn alongside as well.