I've never considered cooking or baking with breastmilk, but then again I have always had supply issues so I've never had extras on hand.
The situation where it would seem to make the most sense is with a baby who has a known allergy or intolerance to cow's milk. But with a child with no issues like that it seems to me not worth the pumping, etc. Espcially given the fact that the antibiodies in human milk break down with excessive heat.
Interesting thought. (My DD, btw, is allergic to milk and soy and every other alternative milk we have tried so if I had several ounces of BM just sitting around I would give it a shot)