Add all the marinade ingredients to the blender jar. Blend at low speed until the achiote breaks into small pieces, then increase to high and blend until smooth.
Put all the pork pieces into the mixing bowl and pour the marinade over it. Toss the meat until it’s thoroughly coated with the marinade. (I used my hands, wearing gloves of course.) Refrigerate for up to 1 day. Bring to room temperature before cooking.
Lay the banana leaves flat and cut any thick edges away. Gently rinse the leaves under cold water and wipe away and white spots. The leaves split easily, so handle with care. Pat dry. Turn on a gas or electric burner. Using tongs, carefully pass each leaf over the heat, until the top of the leaf turns shiny and softens. Do this quickly, in about 5 seconds, so it doesn’t scorch. Flip the leaf and repeat. Use a damp kitchen towel to wipe both side of the leaves to remove any remaining dirt or spots.
Preheat the oven to 350° F/175° C.
Line the baking dish with banana leaves, overlapping them as necessary to completely cover the bottom and sides. Some of the pieces may hang over the sides, and that’s fine.
Transfer the pork and all the marinade into the dish, spreading the pork into an even layer. Fold overhanging leaves onto the pork and cover the top with more leaves, tucking them between the baking dish and the leaves in the dish. Cover dish tightly with foil.
Bake for about 2-½ hours, checking at the 2 hour mark. The pork should be tender enough to pull apart with a fork. Remove from the oven and let stand covered for 10 minutes. Remove the foil and top layer of leaves. Transfer pork and juices to a serving bowl. Coarsely shred with two forks, and serve hot.