Sprinkle the roast with kosher salt and let it rest while you prepare the other ingredients. Process the tomatoes in a blender until pureed. Combine tomatoes, ketchup, vinegar, jalapeno, garlic, cumin and 2 teaspoons salt in the slow cooker. Add the rump roast, fat side up. Add the bell pepper and onion on top. Cook for 4 hours on high or 8 hours on low, until the roast is very tender and pierces easily with a fork.
Remove the roast from the pot and place on a large baking sheet or tray. Add the olive brine, if using, to the juices in the pot and stir.
Remove and discard visible fat from the roast. Cut the roast across the grain into 2 or 3-inch thick slices, then shred the meat with two forks. Return shredded beef to the pot with the tomatoes and other vegetables.
Serve the ropa vieja in a bowl over rice. Top each serving with chopped green olives, if desired.