It turns out your Retina MacBook Pro 15" can run beyond the resolutions offered in it's System Preferences. In researching this fact I found that there are a number of third-party utilities that can be used to go beyond your Mac's offered resolutions.
The following utility from avibrazil is a good one and I recommend it. I've been using it for a few days and it works flawlessly with my MacBook as well as an external display.
The first link is to the project itself and the second link is the installable binary dmg file.