Pies are... tricky. I've been playing around with them and they're currently coded to only allow filling from the same food category: This means I can mix odd things together, like mushrooms, chopped cabbage, turnip gelatin, and... I dunno, cassava. That'll all fit into a pie just fine, but if I used fruit gelatin, that would only work in fruit pies.
I have absolutely no idea how feasible it is for multi-nutritional pies. Maybe we can check it out.
Now you've got me wondering if you can put eggs in pies...