In a medium saucepan, combine 2 1/4 cups of water with a pinch of salt and bring to a boil over moderately high heat. Remove from the heat and stir in the couscous. Cover and let stand for 5 minutes. Fluff the couscous with a fork and transfer to a large bowl to cool.
In a small bowl, whisk together the orange juice, vinegar, mustard and sugar. Whisk in the olive oil in a fine stream. Stir in 1 1/2 teaspoons salt. Pour the dressing over the couscous. Add the scallions, carrots, cucumbers, and jalapeño and toss well. Season with salt and pepper.