While farro cooks, preheat 1 side of a gas grill to very high (500?F and up). Toss together broccoli, 4 tablespoons oil, pepper, and 1 teaspoon salt in a large bowl. Place broccoli and lemons on oiled grates on lit side of grill; grill, uncovered, until broccoli begins to char, 2 to 3 minutes per side. Move both to unlit side, and cover until broccoli is just tender, 2 to 3 more minutes. Remove from grill.