Drizzle the bottom of a 4-quart slow cooker crock with olive oil. Stir tomatoes with juice, seitan, sausage, onion, green bell pepper, celery, vegetable broth, garlic, miso paste, Cajun seasoning, thyme, and oregano into crock.
Cook on Low for 4 hours. Add rice to the crock and cook on High until rice is cooked through, about 30 minutes more. Garnish with parsley.