Preheat the oven to 400 degrees F. Line a rimmed baking sheet with foil and then set a rack on top. Lightly oil the rack.
Toss the chicken and scallions with the soy sauce, sesame oil, 1/4 teaspoon salt and a few grinds of black pepper in a small bowl; let stand at room temperature 15 minutes. Meanwhile, wrap each pineapple chunk in a half slice of bacon and secure it with a toothpick. Arrange them on the rack set in the baking sheet.
Wrap each piece of chicken and 2 pieces of scallion with the remaining bacon, securing each with a toothpick and arranging on the rack.
Bake the chicken and pineapple rumaki until the bacon is crisp, 20 to 25 minutes. Serve the rumaki on a platter, sprinkled with the sesame seeds.