For the pineapple-rum glaze, roughly chop pineapple. Transfer to a food processor and pulse until smooth. Pass pineapple puree through a fine-mesh sieve into a large skillet (use a rubber spatula to help push the puree through). Discard remaining pulp. (If using pineapple juice substitution, just pour into large skillet.)
Add rum, vinegar, crushed red pepper and a pinch of the seasoned salt to skillet and bring mixture to a simmer over medium-high heat. Simmer, whisking frequently 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 backbone. Flip chicken, breast-side up, and press down firmly on the breastbone to flatten it. Cut wings off at the third joint (at the wing tip). Line a plate with paper towels and place the chicken on it skin-side down to dry. Season chicken all over with 1 tablespoon of the seasoned salt.
Place chicken in grill basket with a handle, breast side up. Place basket on grill, making sure the handle of the basket is not resting on the grill grates. Close grill. Grill chicken over medium heat, undisturbed, until the skin starts to brown; 30 to 40 minutes. Reduce heat to medium-low and brush skin side with some of the remaining glaze. Flip chicken in basket. Baste back side of the chicken and close grill. Grill, with lid closed, 10 to 20 minutes longer (or until deepest part of the thigh registers 165°F), basting and flipping every 4 to 5 minutes.
During last 5 minutes of grilling, toss scallions with vegetable oil and a large pinch of seasoned salt. Grill, turning as needed, until charred and tender; about 5 minutes. Chop scallions, transfer to a bowl and add the olive oil, lemon juice and a pinch of seasoned salt. Mix well.
Transfer chicken to a serving platter and serve with the grilled scallion relish and reserved glaze.