Why would you want to do it this way?
Surely if you want to do something that needs a GPU then having it switch off beacause your on battery kind of defeats the point of having a laptop.
Now I can see the point of having a chipset that only powers the GPU when its features are being used. If I am logged on and checking my mail then I dont really need my GPU so no need to have it running. However once I have checked my mail and want to game for 20mins or so whilst I'm waiting for my flight then fire up my GPU.