Odd, normally these modes just follow a simple norm.....
You might to try a monitor thats one size bigger (the usual 21" that will go to 1680x1050) and use the 1440x900 mode defined there.
Those things can be tricky .... just reminds me when I tried to get a 24" 16:10 running under 1.4.x (which only had 4:3 monitors predifined).
If your monitor has an analoge input (VGA or via DVI-VGA adapter) you might try it there 1st as most monitors are bit less picky on that side.