Separate the pulp and skins of the scuppernongs. Add the skins to a medium saucepan; add the pulp to a small saucepan. Add 1/2 cup water to each saucepan and bring to a boil. Add the sugar to the skins (no sugar to the pulp). Reduce the heat and simmer both pots until the pulp starts to break down, 5 to 6 minutes. Remove the pulp from the heat and strain into the skins, pushing as much juice through the strainer as possible; discard any solids (and seeds). Add the baking soda to the mixture; it will turn green and foam up. Don't worry--just continue to stir and simmer.
In a bowl, combine 1/4 cup water and the cornstarch; mix until there are no lumps. Add the cornstarch mixture to the filling and bring back to a boil. Stir in the margarine until melted. Remove from the heat.
Roll out one of the pie crusts and transfer it to a 9-inch pie pan. Pour the filling into the uncooked crust. Roll out the second crust and set it on top of the pie. Cut vents in the top crust in several places. Brush with beaten egg, using a pastry brush.