Prepare a medium-hot grill fire. In a small bowl, whisk the jelly, tequila, orange or pineapple juice, and orange zest. Generously season the pork with salt and pepper. Coat the pork and the peaches with a thin film of the olive oil.
If using charcoal, bank the coals so that one side of the grill is cooler. Grill the pork over the hotter side of the grill, turning it until all sides develop grill marks, about 2-1/2 min. per side. Move the pork to the cooler side of the grill (on a gas grill, lower the heat to medium or medium low) and brush the pork all over with the glaze. Cover the grill or set a disposable aluminum pan over the pork. Grill for 5 min. and then turn once and brush again with the glaze. Cover and continue grilling until the pork's internal temperature reaches 145°F, about another 5 min. Brush the pork with the glaze again, transfer it to a clean cutting board, and cover it loosely with foil to rest for about 5 min.
Meanwhile, grill the peaches, cut side down, over the hotter part of the grill (on a gas grill, raise the heat to medium high) until grill marks appear, 3 to 4 min. Turn the peaches over and brush with the glaze. Continue grilling until warmed through, about another 3 to 4 min.
Carve the pork into 1- to 2-inch slices and arrange on a platter with the peaches. Sprinkle lightly with salt, drizzle with any leftover glaze and juices from the pork, and serve.