1. Combine stout, vinegar, lemon zest, lemon juice and salt in a small, heavy-bottomed saucepan. Bring to a boil over high heat, then reduce heat to medium and simmer vigorously until reduced to about 1/4 cup syrup, 20 to 25 minutes.
2. Meanwhile, build a fire on one side of a grill. When fire has died down, and coals are medium-hot (you can hold your hand 5 inches above grill grate for about 2 seconds), move enough coals to other side of grill to form a single layer, leaving most coals on hot side.
3. Rub lamb with oil, sprinkle generously with salt and pepper, place directly over hot part of grill and sear well, about 2 minutes a side. Move chops to cooler side of grill. Brush side facing up with glaze and cook for 4 minutes; turn over, brush other side generously with glaze and cook about 4 minutes more for medium-rare. To check for doneness, make a 1/4-inch cut in thickest part of one chop; it should be slightly less done than you like it.
4. Remove chops from grill, cover them loosely with foil, and let rest for 5 minutes. Brush again with glaze just before serving.