Line an 8-inch square nonreactive baking dish with plastic wrap. Puree tomatoes and juice, vodka, hot sauce, lemon juice, salt, peppers, Worcestershire sauce, and 1/2 cup water in a blender. Pour into prepared baking dish. Chill in freezer, scraping every 30 minutes with fork, until frozen, 2 to 3 hours.
Shuck oysters, leaving them on the half shell. Arrange on a bed of crushed ice. Top each with 1 teaspoon granita, and serve immediately garnished with horseradish and celery leaves.