Charlie, I've used the cream substitute, I think it was called 'Farm Fresh' in making pastry cream, and it was just fine. Can you use eggs in the recipe? If so, this is the recipe I use for a coconut cream pie, without the coconut.
Cream pie filling
2/3 cup sugar 1 cup milk 1Tbutter
1 ½ T flour 1tsp. vanilla 2 egg yolks
Beat egg yolks lightly and stir in sugar; add milk and butter; beat flour into ¼ cup milk or water to smoothness, and stir into mixture; place in a double boiler and cook over hot water until quite thick.
Pour into a bowl, and let it cool; cover with some Saran wrap to prevent 'skin' forming. Put in a baked pie shell, and add your fruit.
If you can't find the Farm Fresh, try one of the non-dairy creamers in the dairy shelf.