What can I substitute for butter in a custard-based pie?
Butter in this type pie is there to add "richness". It is possible to leave it out entirely and save about 800 calories, but it may not satisfy your definition of a good dessert. If you reduced the amount to 1/2 a stick (1/4 cup), you would reduce the total calories by about 400 and still have a very nice pie. 

I would not recommend replacing the butter with a reduced calorie margarine or spread, as they contain water and other stuff that will not improve the pie.