It seems that it is possible to change resolution to 1920x1200, by doing hard drive install.
But, I'd love to be able to use this resolution without writing anything to my hard drive.

I am able to use screen=1600x1200, but cannot get any widescreen resolution to work, like screen=1920x1200 or screen=1400x1050.

Dell Inspiron 9300, WUXGA 1920x1200 monitor.
(In Ubuntu Live, and Windows, 1920x1200 works fine. But I like Knoppix )

Here's a similar request in an old post, where hard drive install solution was given: http://www.knoppix.net/forum/viewtopic.php?t=5489