I got this working, so I thought I would post an FYI for anyone else who might want SCSI drivers other than the ones the script installs by default.

The knx-hdinstall script is smart about only loading those SCSI drivers that are necessary for your particular machine into the initrd file. The trouble is that the SCSI devices it works with are not the full range that Knoppix supports, but just those that are included in the miniroot. So my solution was to create my own modified miniroot with the device driver I needed, and change the knx-hdinstall script to use my miniroot. There may be easier fixes, but this worked for me.

Here is what I did from a root prompt:

1) cp /usr/local/bin/knx-hdinstall /tmp
2) mkdir /mnt/work
3) mount -t vfat -o loop,ro /cdrom/KNOPPIX/boot.img /mnt/work
4) gzip -cd /mnt/work/miniroot.gz > /tmp/miniroot
5) umount /mnt/work
6) mount -t ext2 -o loop /tmp/miniroot /mnt/work
7) cp /lib/modules/2.4.22-xfs/kernel/drivers/scsi/aacraid/aacraid.o /mnt/work/modules/scsi
(you'll want to substitute your own device driver here)
8) umount /mnt/work
9) Edit knx-hdinstall and change:
Code:
      mount -t vfat -o loop,ro /cdrom/KNOPPIX/boot.img $TL1
      gzip -cd $TL1/miniroot.gz > $TR/miniroot
      umount $TL1
to:
Code:
      cp /tmp/miniroot $TR
10) rmdir /mnt/work
11) /tmp/knx-hdinstall

That should do it. When working with your own driver, make sure you get any dependencies it may have. I'm no module guru but I think the simplest way is using lsmod. If you don't want to trust that you can use depmod -n > /tmp/modlist.txt and check over the file for your driver.

Hope that helps anyone in a similar situation.