How can I force specific HiDPI modes?

Solution 1:

After a lot of messing about I managed to get this working in two ways.

1. The first is by using the Retina Display Manager I found on reddit. This gives a menu with all the available display modes, but try the second method first.
   Screenshot 1 (Warning, large file)
2. The second I found after using Retina Display Manager, to my surprise the Displays pane in System Preferences was reporting it was running in 720p mode.
   This mode is actually 1280x720 HiDPI (2560x1440).
   Screenshot 2 (Warning, large file)

Yay!

I was going to embed screenshots of these new menus, but they are now insanely huge due to HiDPI mode, so I just linked to them instead.