For the Cajun seasoning: Combine the garlic powder, oregano, paprika, allspice, if using, 1 teaspoon salt and 1/2 teaspoon black pepper.
For the salad: Combine 1 teaspoon Cajun seasoning with the rice and cook according to the rice package directions. Set aside in a bowl.
Meanwhile, in a large microwave-safe bowl, combine the shrimp, lemon juice, oil and 2 teaspoons Cajun seasoning; cover the bowl with plastic wrap, leaving a 1-inch opening for a vent. Microwave at 80-percent power, stirring once per minute, until the shrimp are opaque, about 3 minutes. Add the shrimp and the cooking liquid to the rice. (Save the remainder of the Cajun seasoning for another use.)
In a small saucepan, bring the okra and 1/2 cup water to a boil; cook until crisp-tender, 3 minutes. Add to the shrimp and rice.
Stir the bell pepper and parsley into the rice salad and serve at room temperature.
This salad is even better the second day, but the okra will lose its brilliant green color from the acidic lemon juice in the dressing. If you plan to make it ahead, keep the okra separate from the salad and add it before serving. Leftover Cajun seasoning can be used on scrambled eggs, fish, roasted tomatoes or any veggies.