Using tongs, briefly toast the guajillo chiles over an open flame or in a cast-iron skillet until fragrant, about 5 seconds per side. Transfer the chiles to a blender. Add 2 cups of the hot water and let stand for 15 minutes.
Add the garlic, chipotles, black pepper, cinnamon and oregano to the blender and puree until smooth. Strain the sauce into a large enameled cast-iron casserole, pressing hard on the solids. Add the pork to the casserole along with the remaining 6 cups of water and bring to a boil. Cover partially and simmer over low heat until the pork is tender, about 2 hours.
Using a slotted spoon, transfer the pork to a bowl and let cool slightly. Boil the sauce until it is reduced to 4 cups, about 20 minutes.
Shred the pork with 2 forks and return it to the sauce. Simmer uncovered until the sauce is reduced and just coats the pork, about 20 minutes. Season the pork with salt and let cool slightly.
In a large bowl, stir the masa harina with the water until evenly moistened; let the masa cool. In the bowl of a standing electric mixer fitted with the paddle, beat the lard with the baking powder and salt at medium speed until fluffy, about 3 minutes. With the machine on, add the masa in golf-ball-size lumps, then drizzle in the chicken stock and beat until completely smooth. Increase the speed to high and beat the masa until fluffy, about 3 minutes; the texture should resemble soft hummus.
Using a sharp knife, cut off the stringy tops and bottoms of each banana leaf. Carefully cut the leaves into 8-inch squares; set aside any with tears to use for steaming. You'll need about 18 squares.
Spread 1/4 cup of the masa in a slightly off-center 4-by 6-inch rectangle on a banana leaf square. Spread 2 tablespoons of the braised pork over the masa. Fold the bottom edge of the square up and over so that the masa encloses the pork filling. Fold the top edge of the leaf down, then fold in the sides to close the packet. Tie the tamal with kitchen string. Repeat to form the remaining tamales.
Line a large steamer with a layer of banana-leaf scraps. Add the tamales in 2 loose layers. Steam the tamales over boiling water for 1 hour and 20 minutes, replenishing the boiling water as needed. Remove from the heat and let the tamales stand in the covered steamer for 30 minutes. Transfer the tamales to a platter and serve, passing scissors at the table.