In a medium bowl, whisk the wasabi powder with the water, then whisk in the mayonnaise and sugar.
In a large bowl, toss together the cabbage, carrot and ponzu. Add 1/4 cup of the wasabi mayo and toss to coat. Season the slaw with salt.
Light a grill or preheat a grill pan. Cut a cross pattern into the ends of each hot dog. Grill over moderately high heat until lightly charred all over, about 4 minutes. Place a hot dog in each bun and top each with 1 teaspoon of the remaining wasabi mayonnaise. Spoon about 1 cup of slaw onto each hot dog, sprinkle with the sesame seeds and serve.