Whisk all marinade ingredients in a large dish. Add lamb, turning twice to coat completely. Marinate for at least 6 hours, but preferably overnight.
When you are ready to grill it, remove lamb from marinade and pat dry. Thread 2 or 3 metal skewers through the meat to keep it flat while cooking. Spread tapenade over both sides of the meat.
Brush barbecue rack with olive oil. Place lamb 4 inches above hot coals and cook for 5 minutes. Turn meat and cook on other side 5 minutes. Turn twice more every 5 minutes, basting with tapenade.
Raise grill rack to 6 inches and cook 20-25 minutes more.
Remove lamb from heat, let rest 10 minutes before slicing.