In a saucepot over medium heat, add the butter, red onion, shallots and garlic. Cook for 3 minutes until the onions are soft, but not brown. Add the grilled pineapple, honey, dark rum, pineapple juice and green onions. Bring to a boil, remove from heat and set aside. Season to taste with salt and pepper.
Preheat grill to medium high.
Place the Mahi Mahi on a flat surface and rub with bbq seasoning, pressing the spice gently into the meat. Brush lightly with olive oil. Grill for three to four minutes per side, glazing liberally with pineapple glazing syrup until the fish is just cooked through, tender and flakes lightly.
Serve with coleslaw.