I want to use a HiDPI resolution that is half the native size of the monitor, but this doesn't work
In some cases, you will see a resolution that is half the size of the native resolution of the screen. Selecting it will not work.
If you look in the Console (/Applications/Console), you'll observe a message like this one:
11/11/14 10:35:56,465 WindowServer: CGXCompleteDisplayConfiguration: Monitor 4304241; Mode -1569958512 not available
11/11/14 10:35:56,465 SwitchResX Control: CGSCompleteDisplayConfigurationWithOption
This is a bug within OS X itself. OS X reports this resolution as available when building the resolution list, but doesn't let you select it. There's nothing to do at this point, and SwitchResX cannot avoid showing this resolution in its list.