Enable All HiDPI Modes
The answer is that it is not possible, when the  13" MacBook Air's higest native resolution is 1440x900. It simply does not support 1280x800 or 1440x900 HiDPI modes, since it does not have a retina display or a display with enough pixels to do HiDPI. For the MacBook Air in its current form, it would need to have a LCD display with at least 2880 X 1800 pixels to enable HiDPI modes.

To do HiDPI 1280 x 800 you would need a display with resolution of at least 2560 x 1600, again, not possible on the 13" MacBook Air's display.

As noted in Signs in Mountain Lion point to "retina" display MacBooks sooner than later retina displays and graphics require 2x the resolution of normal graphics and displays.