For the pineapple-rum glaze: Roughly chop the pineapple. Transfer to a food processor and pulse until smooth. Pass the pineapple puree through a fine-mesh sieve into a large skillet (use a rubber spatula to help push the puree through). Discard the pulp. If using the pineapple juice, just pour into a large skillet.
Add the rum, vinegar, red pepper flakes and a pinch of seasoning salt to the skillet and bring the mixture to a simmer over medium-high heat. Simmer, whisking, until the mixture is bubbly and reduced to about two-thirds its original volume, about 5 minutes. Reserve 1/4 cup of the glaze in a small bowl.
For the chicken: Use kitchen shears to cut out the backbone from the chicken. Flip the chicken, breast-side up, and push down firmly on the breastbone to flatten it. Cut the wings off at the third joint (at the wing tip). Line a plate with paper towels and put the chicken on it skin-side down to dry the skin. Season the chicken all over with 1 tablespoon seasoning salt.
Prepare a grill for medium direct-heat cooking.
Put the chicken in a grill basket with a handle and lock the basket. Put the chicken on the grill breast-side up. Make sure the handle of the basket isn't resting on the grill grates, then close the grill. Grill the chicken, undisturbed, until the skin starts to brown, 30 to 40 minutes. Adjust the grill heat to medium low and brush the skin side with some of the glaze. Wrap a towel around the basket handle or use an oven mitt and flip the chicken over. Baste the back side of the chicken and close the grill. Continue to grill the chicken, basting, flipping and closing the grill every 4 to 5 minutes, until the deepest part of the thigh registers 165 degrees F, 10 to 20 minutes more.
During the last 5 minutes of grilling, toss the scallions with the vegetable oil and a large pinch of seasoning salt and grill, turning as needed, until charred and tender, about 5 minutes. Chop the scallions, transfer to a bowl and add the olive oil, lemon juice and a pinch of seasoning salt. Mix well.
Transfer the chicken to a serving platter and serve with the grilled scallion relish and the reserved glaze.
Reserve some of the basting glaze to use for topping the chicken or as a dip at the table.