EDIT: I got it to work. I did something wrong. *shrug*
I've tried using the gui files stated for 1920x1200, but all it does is shift the gui to the left side of the screen in a 4:3 ratio. In other words, it doesn't display the gui elements on the far right, but about 1/4 from the right side, in the "middle" of the display.
Widescreen mode works great, however.