Meanwhile, spoon 1/2 tablespoon salad dressing over each chicken breast, and coat thoroughly. Sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper. Let stand until grill is ready.
In a medium bowl, toss squash slices with oil until coated on all sides. Sprinkle with remaining 1/4 teaspoon salt, 1/4 teaspoon pepper and the thyme; toss again to combine.
Arrange chicken breasts and squash slices in a single layer on grill. Cook squash just until partially softened, 2 to 3 minutes per side. Transfer to a plate. Cook chicken until juices run clear when pierced with a fork and an instant-read meat thermometer registers 170F when inserted into the thickest part of breast, 8 to 10 minutes per side. Transfer to a platter. Let both stand for 15 minutes.
In a small bowl, combine remaining 1 tablespoon salad dressing and the ketchup. Set aside.
Using the divided containers, place a chicken breast on one side of each container. Spoon 1/2 tablespoon of the tomato mixture over each. Place 4 to 5 slices of squash in the other section of each container, and top each with a piece of butter. Seal, and freeze. (The chicken can be stored, frozen, for up to 1 month.).
When ready to serve, slightly open one corner of container to vent. Microwave on high until hot, 4 to 5 minutes. Let stand in microwave for 1 minute before serving. Garnish with thyme sprigs, if desired.