How can I change DPI on Mac OS X?

Simple question, can't find a single simple answer on the entire internet. Although from what I have read, this might be a driver or hardware limitation.

My problem

I have a 1440 x 900 native pixel resolution. I want to run at a 1280 x 800 scaled resolution.

I know you can change this in the display settings, but then you see a horrible blur because of the hardware upscaling. OS X has built it DPI upscaling, and has been resolution independant for a while now. Why can't I change the perceived resolution at the software level?

My attempts

I've force enabled HiDPI modes via Quartz Debug and it has given me one HiDPI mode. 720 x 450 PX @ 2 DPPX. I've installed SwitchResX and although I can add custom resolutions, I can't force it to use the graceful scaling I want, and I can't see a way to manage the DPI or DPPX.

To summarise

I want to run my native MacBook Air 13-inch display at 1440x900 PX and have Mac OS X upscale to this from 1280x800 PX @ 1.125 DPPX, still 128 DPI, using the same DPI scaling technology that makes "retina" displays look so clear.

I would love this option but I appreciate it may not be possible. I love my Macbook Air and don't want a rMBP, but I also have terrible eyesight and would prefer a 1280 width, but not with the horrible upscaling that is currently my only option making me squint even more.

Thanks.