Preheat a grill to medium-high heat.
Mix the mango, red onion, rice vinegar, scallions, cucumber and jalapeno in a small bowl. Season with salt and pepper.
Grill the hot dogs, flipping them halfway through so both sides get grill-marked and a little charred, until heated through, about 8 minutes.
Brush the inside of the brioche buns with butter. Grill until lightly toasted, 2 to 3 minutes.
Spread both sides of the inside of the buns with mayonnaise to your liking. Place a hot dog on each bun and spoon a few tablespoons of the mango mixture on the dog. Sprinkle with fried onions.