Drain the soaked chiles and put them in a blender with the marjoram, oregano, garlic, 1/4 of the chopped onion and the pineapple juice and puree until smooth. Add the salt and pepper. Mix to combine.
Put the pork in large bowl and pour the marinade on top. Using a fork, mix well. Cover with plastic wrap and refrigerate for 30 minutes.
Heat a grill pan over medium-high heat.
Divide the pork mixture into 8 equal portions. Shape each portion into a 1/2-inch thick patty.
Oil the grill pan. Working in batches, grill the burgers until cooked through, 6 to 8 minutes per side. Transfer the burgers to a plate and cover with aluminum foil to keep warm. Let rest for about 8 minutes.
Meanwhile, brush the grill pan with oil and grill the pineapple slices until golden brown, 2 minutes per side.
Spread 1 teaspoon mayonnaise on the cut sides of each bun and warm on the grill pan about 2 minutes, cut-side down.
Place a burgers onto the bottom half of the bun. Top with a slice of grilled pineapple, some cilantro and the remaining chopped onion. Cover with the bun tops and serve.