Combine all the mayonnaise ingredients in a small bowl. Refrigerate until needed.
Combine all remaining ingredients, except plank, in a bowl. Let scallops marinate for 20 minutes. Preheat barbecue to medium-high. Set the plank on the grill. Close the lid and heat the plank 3 to 4 minutes. Carefully set the scallops on the plank. Turn the heat underneath scallops to medium-low, leave the other side set to medium high. Close the lid and cook scallops 8 to 10 minutes, or until just cooked through. Serve scallops directly from the plank with the flavoured mayonnaise alongside.