Combine buttermilk, tomato ketchup, light soy sauce, ground ginger, pepper, salt, and barbecue seasoning in a bowl to make the marinade.
Place the beef rump steaks in the marinade, cover, and refrigerate for at least 8 hours, overnight, or up to 2 days (longer marinating makes the meat more tender).
Remove the steaks from the marinade and pat dry with paper towels.
Preheat a grill or griddle to high heat. Grill the steaks for 4-5 minutes on each side until they reach an internal temperature of 63°C (145°F) for medium doneness.