If your video card doesn't report the VESA mode you want as available, then it isn't. At least not to Chameleon.
Which resolutions are available when an OS is running with video drivers loaded is another matter entirely.
I've seen posts from someone who got more VESA modes by updating or crossflashing the BIOS on their video card.
Visit mvktech.net and see if you can find a newer BIOS version.
Someone else reported once that more VESA modes became available when using the display output that's closest to the motherboard.
If that's a VGA port on your card and you're using DVI right now, try the VGA port with a converter and see if your native resolution becomes available.
There was also a thread going on somewhere about patching the video card BIOS to enable more VESA modes.
/EDIT - here:
http://forum.voodooprojects.org/index.php/topic,1227