Place the avocado in a bowl with the lime juice and gently toss to coat. Mix in remaining ingredients, except oysters. Store salsa in the fridge until the oysters are ready. Preheat the barbecue to medium. Set the oysters directly on the grill. Close the lid and cook until the oysters just open, about 5 minutes. Transfer to a platter and cool until they are safe to handle. With an oyster knife, carefully remove the top shell. Top each oyster with a spoon of salsa and serve. Any leftover salsa can be served with taco chips.