Preheat barbecue to high. In a large aluminum roasting pan, combine tomatoes, cooking sauce, coconut milk and lime juice. Season lightly with salt and pepper.
Place mussels and clams in a single layer in pan; cover tightly with foil. Place on grill, close lid and cook until shellfish open, 12 to 15 min. Remove roasting pan from grill. Discard any closed shellfish.
Spoon mussels and clams onto a large serving platter or into a bowl; spoon sauce overtop and garnish with green onions.