Place the soy sauce, mirin and maple syrup in a bowl and stir to combine. Add the chicken and green onion and toss to coat. Cover and set aside for 5 minutes to marinate.
To make the cucumber salad, place the vinegar, vegetable and sesame oil, mirin and cucumber in a bowl and mix to combine. Set aside.
Thread the chicken and green onion onto skewers, reserving the marinade. Heat a char-grill pan or barbecue over high heat and char-grill or barbecue the skewers for 3 minutes each side or until cooked through. Place the reserved marinade in a small saucepan over high heat and cook for 2 minutes or until heated through and thickened slightly. Serve the chicken with the cucumber salad, yakitori marinade and wasabi.