Preheat the broiler. Halve the poblanos lengthwise and remove the stems and seeds. Place cut-side down on a baking sheet and broil until browned in spots, about 5 minutes. Transfer to a bowl, cover with plastic wrap and set aside 5 minutes. Peel off the skin, then roughly chop.
Preheat the oven to 375 degrees F. Combine the lard, baking powder and 1 teaspoon salt in a large bowl and mix with a wooden spoon until combined. Whisk the masa harina, 2 cups hot water and the broth in a separate bowl; add to the lard mixture and mix until smooth.
Heat the vegetable oil in a large skillet over medium-high heat. Add the scallions, garlic, chipotles, cumin and orange zest and cook, stirring, until the scallions are soft, about 2 minutes. Add the plantains, roasted poblanos and capers and season with salt.
Gently stir the scallion mixture into the masa mixture. Line a 9-by-13-inch baking dish with the banana leaves, crossing one over the other, and add the filling. Fold the banana leaves over the filling, tucking them in to seal. Cover with aluminum foil and bake until the filling is set, about 1 hour, 15 minutes, uncovering halfway through.