Preheat a grill to medium. Combine the frozen rice, scallions (white and light green parts only), celery, garlic, bell pepper, sausage, tomatoes, okra, olive oil, Cajun seasoning, 1 teaspoon salt and a few grinds of pepper in a large bowl; toss well.
Lay out four 18-inch-long sheets of nonstick heavy-duty foil on a work surface. Divide the rice mixture among the foil sheets. Bring the two short ends of the foil together and fold to seal, then fold in the sides and seal to make flat packets.
Grill the foil packets, flipping once, until the vegetables are tender and the rice and sausage are hot, about 8 minutes per side. Carefully open and top with the dark scallion greens.
Precooked frozen rice is ideal for foil packets — you don’t even need to thaw it. We used nonstick foil to keep the grains from sticking.