How can I set the resolution of my external display to 1280x800?

Hi, I'm using a Wacom Cintiq 12WX external pen tablet display with a 24" iMac.

The recommended resolution for this Cintiq is 1280x800. But when I go to System Preferences>Displays set the resolution of the display, the only available options for "Wacom Cintiq 12WX" are:
640x480
800x600
832x624
848x480
1024x768

1024x768 is my nearest option, but it has a different proportion than the display's physical shape--resulting in a slightly stretched display. Because I work with graphic design, this is not exactly desirable...

None of the other resolutions have the same proportion as 1280x800, so a stretch seems inevitable; not even working with a smaller resolution will give me the desired ratio.

So my question is, is there any way to choose resolutions other than those listed in the "Display tab" of System Preferences?

I also noticed that I have the option to rotate the tablet screen 90 degrees; when I do this, the option 800x1280 becomes available, but of course it's sideways...

Any help at all would be very greatly appreciated!