Heat a gas grill to medium-hot, or prepare a medium-hot fire in a charcoal grill.
Combine the vinegar, honey, mustard, and oil in a warmed bowl and whisk together with a fork (heating the bowl makes the honey mix more easily with the other ingredients). Pour half the vinaigrette into a small serving bowl. Set aside. Toss the scallops with the remaining vinaigrette, remove them from the bowl, and season with salt, pepper, and thyme. Season the oysters with pepper only.
The grilling has to be done very quickly. Have a 3-foot-long sheet of aluminum foil ready. Grill the scallops for 30 seconds on one side only. Place the oyster shells on the grill — try to balance them so they don’t lose any of their brine. Turn the scallops, throw the black peppercorns onto the hottest part of the fire, cover the shellfish tightly with foil to trap the smoke, and step back. Let the scallops and oysters cook for about 2 minutes, then remove them and serve immediately — the scallops with the remaining vinaigrette, the oysters with the lemon wedges.