In a large, heavy saucepan, whisk together the egg yolks, 3/4 cup sugar, 1 teaspoon nutmeg, and citrus zests. Stir in the milk and 2 cups of the cream. Place over low heat and cook, stirring constantly with a rubber spatula, just until the mixture thickens enough to coat the spatula and a finger run along its length leaves a track, or an instant-read thermometer reads 160°F, about 20 minutes.
Immediately remove from the heat and strain through a sieve into a pitcher or bowl. Cover and refrigerate to chill thoroughly, about 1 hour.
Just before serving, in a bowl, whip the remaining 1 cup cream with the 2 tablespoons sugar until soft peaks form. Pour the chilled custard into a punch bowl and stir in the liquor, if using. Dollop the whipped cream on top and sprinkle with nutmeg.