Preheat a grill to medium-high heat. Brush the grill with oil and place the pineapple rings flat on the grill, and cook, flipping once, until charred on both sides, about 3 minutes. Set aside to cool while you mix the cocktail.
In a large pitcher, add the jalapeño, cilantro and 2 tablespoons of the brown sugar. Using a muddler or the handle of a wooden spoon, begin to muddle or crush the ingredients together. Chop all but 2 pieces of the pineapple (will be used for garnish) into 1-inch pieces and muddle those with the jalapeño mixture. Add the tequila, pineapple juice, lime juice and syrup and stir well to combine. Let sit for at least 10 minutes or place in the refrigerator until ready to serve.
In a small saucer, combine the remaining sugar, salt and chile powder. Dip the rims of the margarita glasses into another saucer with water to wet the rims, alternately rub rims with half a lime, then dip into the sugar, salt and chile mixture. Fill each glass with the chunky margarita (making sure you are adding the chunks of muddled fruits and vegetables) and garnish with a wedge of pineapple.