In medium saucepan, bring 2 inches water to boil over high heat. Place steamer basket in saucepan, add tempeh, cover and steam 20 minutes.
Meanwhile, in large bowl, mix tamari, wasabi powder if using, and sesame oil. Add hot tempeh and mix well to coat. Set aside to marinate while preparing rice.
In medium saucepan, bring 2 cups water to a boil. Add rice and salt and return to a boil. Cover, reduce heat to low and simmer until water is absorbed, about 35 minutes.
Preheat oven to 375°F. Remove rice from heat and let stand, covered, 10 minutes. Transfer rice to large bowl and fluff with fork.
Drain tempeh; spread on baking sheet. Bake, stirring occasionally, until lightly browned on all sides, about 25 minutes.
Bring small saucepan of water to a boil. Add corn, cover and cook 2 minutes. Drain, rinse under cold running water and drain well. Set aside.
In small bowl, whisk together vinegar and mirin. Gradually pour mixture over rice while still slightly warm and mix gently to combine. When rice reaches room temperature, add corn, mushrooms, radishes and tempeh and toss to blend.
Break thick stems off watercress; discard. Break remaining watercress in half, wash, then dry thoroughly in salad spinner or with paper towels. In large bowl, toss watercress with Peanut-Miso Dressing.
Place rice salad and watercress on individual serving plates; garnish with carrots and daikon. Alternatively, if salad is part of a buffet, place rice salad and watercress side by side on large serving platter; arrange carrots and daikon on separate plate.