In a small bowl, combine gelatin and cold water; set aside to soften. Combine milk and 1/2 cup sugar in a small glass bowl. Microwave for 3 minutes or until milk begins to boil.
In a steady stream, pour hot milk into beaten eggs, whisking constantly. Cook custard in the microwave, whisking after each minute, until it thickens slightly, about 2 minutes. Stir in softened gelatin and mix until melted. Stir in rum.
Let filling cool to room temperature. (If you refrigerate it, stir occasionally to prevent gelatin from setting too soon. See Editor's Note.)
In a medium bowl, whip cream until soft peaks form; beat in 1/4 cup sugar.
Set aside 1/2 cup of whipped cream for garnish; keep refrigerated. Fold remaining whipped cream into eggnog mixture. Pour filling into pie crust and refrigerate 4 hours or overnight.
Garnish with reserved whipped cream and a dusting of nutmeg.
If the custard sets in the refrigerator before you've added the whipped cream, you'll need to gently reheat the custard until the gelatin melts. Let cool to room temperature, then fold in whipped cream.