This may be helpful to you.
I was able to run 1936x1200 (monitor rates were not happy with 1940x). 1280x768 didn't quite work.. but I did not have too long to test alot of refresh rates.
You may not be aware of this, but X can load with virtual resolutions that are larger than what the actual hardware displays. When you use that feature you can scroll the desktop area around the virtual space. This could be used to create any mode, even if the exact mode was not shown by the monitor (for instance, I did load 1940x1200 and scroll those 4 pixels).
The option used (in the screen section of the X config) is:
Virtual xsize ysize