Spread the cucumber-yogurt sauce inside the pita first so you get a cool contrast with every bite of the spiced meatballs. Even if you're tempted to use nonfat Greek yofurt, don't. The 2% yogurt is what gives the sauce its satisfying flavor and texture. If you're wary of slicing red cabbage, you could always use a store-bought coleslaw mix, however the color won't be red (and not nearly as pretty).
Combine meatballs and stock in a microwave-safe bowl; cover with plastic wrap. Microwave at HIGH 2 minutes or until meatballs are warm.
Wrap grated cucumber in a clean kitchen towel, squeezing out as much liquid as possible. Combine cucumber, yogurt, onion, 1 1/2 teaspoons juice, dill, 1/4 teaspoon salt, pepper, and garlic in a bowl, stirring with a whisk. Combine remaining 1 1/2 tablespoons juice, remaining 1/8 teaspoon salt, cabbage, and oil in a bowl. Divide yogurt mixture among pita halves; top evenly with cabbage mixture, tomatoes, meatballs, and parsley leaves.