Preheat oven to 350º. Slice off top of bell peppers. Discard stems and remove ribs and seeds. Arrange peppers upright in a large casserole dish.
In a large skillet over medium heat, heat oil. Add chicken and oregano; season with salt and pepper. Cook, turning occasionally, until chicken is golden brown and no longer pink, about 8 minutes per side. Transfer to a cutting board and let rest 5 minutes, then finely chop.
In a medium saucepan over medium-high heat, bring 1 cup stock to a boil. Remove pot from heat. Stir in couscous. Cover pot with a tight-fitting lid. Let sit until couscous is tender and liquid is absorbed, about 5 minutes.
Transfer couscous and chicken to a large bowl. Add onion, zucchini, garlic, tomatoes, olives, dill, and ½ cup feta; season with salt and pepper and toss to combine.
Stuff couscous mixture into peppers. Sprinkle with remaining ½ cup feta. Pour remaining ½ cup stock into baking dish (to help peppers steam) and cover with foil.
Bake peppers until tender and cheese is melty, 42 to 45 minutes. Serve with lemon wedges alongside.
