Preheat oven to 350°.
Combine first 7 ingredients in a small saucepan. Bring to a boil; reduce heat, and simmer until reduced to 1/2 cup (about 10 minutes). Remove from heat; stir in margarine and pepper. Keep warm.
Cut squash lengthwise into quarters, discarding seeds and stringy pulp. Place squash, cut sides down, on a foil-lined baking sheet. Bake at 350° for 30 minutes or until tender. Cool slightly. Scrape inside of squash with tines of a fork; drizzle sauce over squash. Garnish with tarragon, if desired.