Is there a way to force "low power" graphics? Is there a plist or hidden config to set my graphics/energy preference to "Always use low power/ integrated graphics"?

Seems like energy saver preferences only allow choosing between: 


Smart auto graphics switching
Always use high-powered graphics
gfxCardStatus is an unobtrusive menu bar app for OS X that allows
  MacBook Pro users to see which apps are affecting their battery life
  by using the more power-hungry graphics.


https://gfx.io

By this way you can simply disable your graphic card by an icon in your menu bar.