Preheat the oven to 425 degrees F. Set a rack on a rimmed baking sheet and coat with cooking spray. Toss the chicken with the olive oil in a large bowl, then add the falafel mix and toss to coat. Arrange the chicken on the rack and bake until golden and cooked through, about 10 minutes. While the chicken is baking, stack the pita halves and wrap them in foil; warm in the oven, about 5 minutes.
Meanwhile, mix the hummus, lemon zest and juice, harissa and 3 tablespoons water in a large bowl. Remove 2 tablespoons of the hummus sauce and reserve. Add the coleslaw mix, radishes and parsley to the remaining hummus sauce and toss. Season with salt.
Divide the chicken among plates and drizzle with the reserved hummus sauce. Stuff the slaw in the pitas and serve with the chicken.