Preheat the broiler and position a rack 6 inches from the heat. Preheat a cast-iron grill pan. Brush the scallions with canola oil and grill over moderately high heat, turning, until the scallions are tender and browned in spots, about 3 minutes. Transfer the scallions to a plate. Lightly brush the shrimp with canola oil and place on the grill, shell side down. Drizzle each shrimp with a scant tablespoon of the butter and grill for 3 minutes. Transfer the grill pan to the oven rack and broil for about 1 minute, until the shrimp are cooked through and glazed with the butter. Transfer the shrimp and scallions to plates and drizzle with more of the butter. Garnish with the shredded nori. Serve right away.