Place couscous, cumin, and salt in a large heatproof bowl or storage container and pour 1 cup boiling water on top. Cover tightly and let sit until all the liquid is absorbed, about 10 minutes. If the couscous is not quite tender, add an additional 1/4 cup of boiling water, cover, and let sit for a few minutes longer. Fluff up with a fork. Toss in the beans, corn, onion, cilantro, and jalapeno. Mix in the olive oil and enough lime juice to give the salad a puckery edge. Serve warm or at room temperature.