Prepare the rub by mixing brown sugar, chili powder, garlic powder, thyme, cayenne pepper, and allspice together.
Place each rib rack on extra-wide, heavy-duty aluminum foil (4 inches longer than ribs on either end).
Season racks liberally with salt and sprinkle each rack with 3 tablespoons of the rub.
Turn ribs meat side down and tightly seal each foil pouch. Place on half sheet pan and refrigerate overnight.
Heat oven to 250°F. Combine orange juice and margarita mix in a measuring cup.
Open one end of each pouch and evenly divide the liquid between pouches. Reseal and bake for 2 hours.
Remove ribs, carefully pour braising liquid into heatproof measuring cup. Reseal pouches and refrigerate both ribs and liquid for up to 8 hours.
Remove solidified fat from braising liquid. Transfer liquid to small saucepan and add honey, ketchup, Worcestershire sauce, coffee powder, and cayenne pepper.
Whisk glaze ingredients and cook over medium-high heat until reduced to glaze consistency, approximately 10 minutes.
Heat gas grill to medium-high for 10 minutes. Cut each slab in half and grill flesh side down for 3 minutes.
Flip and cook 3 minutes, then repeat flipping and cooking 3 minutes per side until nicely charred.
Remove from grill, cut into 2-rib portions with kitchen shears. Toss with half the glaze and serve remaining glaze on the side.