In a large saucepan, combine the sugar, cornstarch and 2 tablespoons water until smooth; stir in cranberries. Cook over medium heat until thickened, stirring occasionally, about 5 minutes. Cool for 15 minutes. Pour into crust; set aside.
In a small saucepan, sprinkle gelatin over remaining water; let stand for 5 minutes. Cook and stir over low heat, stirring until gelatin is completely dissolved. Gradually stir in eggnog and rum.
Refrigerate until slightly thickened, stirring occasionally, about 5 minutes. Fold in whipped cream; pour over cranberry layer. Refrigerate until set, about 2 hours. Sprinkle with nutmeg.
This recipe was tested with commercially prepared eggnog.