Found something that helps. If I use "knoppix xmodule=svga" then I get to stare at a black screen for 2-3 minutes while things boot, and eventually, the X desktop appears. If I try to ctrl+alt+F1 to get a text terminal, it just goes black again and I can't ctrl+alt+F5 to get back to X.

The whole time (BIOS screen, boot prompt, black screen while booting, X desktop, and black screen for text terminal) the monitor info screen says "DIGITAL 63.9kHz 60Hz PP 1280x1024" and never changes.

The last things I can see before the long black screen boot process are:
boot: knoppix xmodule=svga
Loading linux...........

Loading minirt.gz..........ready.
(screen clears/blinks once)
Probing EDD (edd=off to disable)... ok

Decompressing Linux... Parsing ELF... done.
Booting the kernel.
(screen clears)