Whisk garlic, rosemary, lemon zest, 3 tbsp. oil, the kosher salt, and pepper together in a small bowl. Spread garlic paste all over pork, then wrap pork in a double layer of plastic wrap, put on a plate, and chill overnight. Mix mascarpone with honey and chill.
Set a large cast-iron skillet on grill and heat grill to medium-high (450°). Brush skillet with 1 tsp. oil and add pork. Cook, covered, until pork is well browned and crusty underneath, 7 to 8 minutes. Turn and cook, covered, to brown all sides, 10 to 15 minutes more, or until an instant-read thermometer registers 150°.
Put peach wedges, meanwhile, in a medium bowl. Scrape vanilla seeds from pod into bowl, then toss in pod. Transfer meat to a cutting board and let rest, covered with foil, 10 minutes. Slice thinly.
While pork is resting, swirl butter in hot cast-iron pan, then add vanilla and peaches in a single layer with tongs. Cook peaches until caramelized on cut sides, 10 to 15 minutes. If your grill is big enough, turn a burner off and set naan over turned-off burner to warm. Otherwise, set peaches aside, covered; then quickly toast naan on grill.
To eat, tear off a piece of naan, spread with mascarpone, and top with a slice or 2 of pork, some peach wedges, and a sprinkle of sea salt.
Make-ahead & packing tips: Marinate pork and make mascarpone honey 1 day ahead. Pack both in a cooler. Cut peaches up to 2 hours ahead, toss with 1 tsp. lemon juice, and pack with vanilla beans and pod; add to cooler.