That won't work. The display itself is going to use the same amount of power. All the pixels are still going to be on whether they're doubled or not. The display doesn't know or care.
Maybe you save a little juice on the GPU side, but maybe not even actually, and I doubt it would matter much.
And to top it all off you have this configurable setting that seems to advertise to the user that the Retina display is a bad thing that drains their battery.
Is the individual pixels really a big part of the power consumption of a display, isn't it mainly the backlight and the gpu? The backlight would stay the same ofcourse, but halfing both the horizontal and vertical resolution of the display would cut the performance requirement of the gpu to a fourth. Isn't that were you would really be able to save power?