Gently peel outer skin from garlic. Cut off top one-third of head, and discard. Place garlic, cut side up, in center of a piece of heavy-duty aluminum foil. Fold aluminum foil over garlic, sealing tightly. Bake at 400° for 40 minutes or until garlic is soft. Remove from oven; let cool 10 minutes. Separate cloves, and squeeze to extract pulp.
Bring chicken broth to a boil in a medium saucepan. Remove from heat. Add couscous; cover and let stand 5 minutes or until couscous is tender and liquid is absorbed. Fluff couscous with a fork, and transfer to a serving bowl.
Combine mayonnaise and next 3 ingredients in a small bowl; stir well. Add garlic; stir well.
Combine couscous, red pepper, parsley, and celery in a large bowl; add mayonnaise mixture, and toss well. Cover and chill 3 hours.