Squeeze the juice from 1 half of the lime into a bowl. Add the olive oil and cilantro and whisk well. Add the mango, red pepper, onion, jalapeno, and lime zest and toss to coat. Gently stir in the crabmeat to keep it from breaking up. Set aside while preparing the avocados.
Cut the avocados in half and remove pit. Squeeze the remaining lime half over the avocado flesh, using the lime half to spread it evenly, in order to keep them from turning black.
Toss the crab salad, check for seasoning, and mound into the avocado halves, dividing it evenly. (Use an ice cream scoop for neat, even rounds, if that's your thing.) Serve right away.
To keep the avocado from sliding around the plate, cut a thin sliver of skin from the rounded side before filling the avocados. That flat portion will help keep the avocado steady on the plate. Variation: For a more dramatic presentation (one more suitable for a sit-down dinner party than a weeknight supper, perhaps) present the salad as a "stack," that is, layer the ingredients in a ring mold (similar to a biscuit cutter without the handle). Instead of serving the salad in the pitted avocado, peel and pit the avocados, then cut them into 3/4-inch dice. Toss them with a little lime juice to keep them from turning dark. Make the crab salad. Place the ring mold over the center of the plate and put the equivalent of half a diced avocado in an even layer in the mold. Tamp it down gently. Top with one-sixth of the crab salad and tamp down gently. Lift the mold carefully and the ingredients will remain in place on the plate.