Remove rind from lemon using a fine metal grater, being careful to avoid the bitter white pith. In medium saucepan, add lemon rind and water; bring to a boil. Stir in couscous; cover. Remove from heat; let stand 5 minutes. Remove lid, and let cook at least 10 minutes.

Meanwhile, roll lemon on countertop to soften pulp. Cut lemon in half and squeeze to yield 2 tbsp juice; remove any seeds. In a small bowl, combine lemon juice, olive oil, mustard, and dill; stir with a fork until blended.

Combine lemon juice mixture and couscous in a large mixing bowl. Fluff couscous lightly with a fork. Add tuna, zucchini, yellow squash, and tomatoes, tossing gently until dressing well distributed. Chill at least 2 hours before serving.