Place pork in a large brining vessel. Cut a few slashes into the fat side of the pork.
Whisk salt, pineapple juice, and water together in a large bowl. Pour brine over the pork. Refrigerate for 12 to 24 hours.
Combine salt, ancho powder, chipotle powder, garlic powder, cumin, cinnamon, and oregano together for the rub.
Remove pork from the brine and place on a deep baking pan lined with foil. Season generously with the spice rub. Reserve remaining spice rub for another use.
Preheat a charcoal grill for 300 to 325 degrees F (150 to 175 degrees C). Place 2 bricks on the grate, if you like, and place the baking pan on top.
Grill, occasionally spooning rendered juices over the meat, until very tender, about 7 hours. Make sure to maintain the same 25-degree range in temperature by adjusting the vents under, and on top of the grill, or smoker. An instant-read thermometer inserted into the center of the pork should read 195 degrees F (91 degrees C).
Remove pan from the grill, wrap top in foil, and let rest for 1 hour. Meanwhile, cook pineapple over the last of the coals until nicely charred, 7 to 10 minutes.
Dice the grilled pineapple to get about 2 cups. Combine in a bowl with serrano pepper, red peppers, cilantro, and rice vinegar. Season with salt and a pinch of the leftover spice rub.
Remove pork bone and pull the meat apart. Add to a bowl with some of the rendered fat; pull apart with 2 forks. Place some pork over each bottom bun, top with pineapple salsa, and cover with the other bun half.