In a medium saucepan, mix sugar, cocoa powder, gelatine, and nutmeg. Stir in 1 cup of the eggnog; let stand 1 minute. Stir over low heat until gelatine dissolves.
Stir in remaining eggnog and rum. Pour into a large bowl. Place bowl in a larger bowl which contains cold water and ice. Stir until mixture mounds on a spoon. Remove bowl from ice water.
Fold in whipped cream. Mound into pastry shell. Chill until firm.
Garnish with sweetened whipped cream and chocolate curls if desired.