The usual method of using Quartz Debug doesn't seem to work anymore.
actually it works on external displays such as TVs, but the list in System Preferences is limited. you can try two things:
- either hold "alt" when clicking "Scaled" button, or
- install one of the menu utilities such as http://www.resolutiontab.com/
the first one didn't work for me as it kept only showing "tv friendly" resolutions (720p, 1080p and 1080i). the second did indeed show 960x540 HiDPI i was after for my tv.