Preheat a grill to medium. Brush the bread, scallions and tomatoes with the olive oil; season with salt and pepper. Place on the grill, cover and cook, turning once, until the bread and scallions are slightly charred, about 5 minutes. Rub the grilled bread with the garlic.
Finely chop the scallions; transfer to a large bowl. Stir in the mayonnaise and vinegar, add the romaine and toss to coat; season with salt and pepper.
Arrange a grilled tomato half on each of 6 plates. Top each with about 1/2 cup of salad, a grilled bread slice, the remaining salad and 2 pieces bacon.