Over medium high heat, with cooking spray, heat a stove-top grill pan or griddle.
In a shallow dish, whisk together grapefruit juice, ketchup, mustard, sugar, chili powder, cumin, olive oil and liquid smoke. Add shrimp and toss to coat. Place shrimp on the grill and cook 2 minutes per side, or until bright pink and cooked through. Serve half of the shrimp over the rice and top with scallions. Reserve the remaining shrimp for po' boy sandwiches, if desired.