Ok, I'm back with a simpler method for a KNOPPIX splitted in two.

Here's the new minirt: ftp://belyi.ac93.org/pub/KNOPPIX/minirt26-knxapp.gz
The linuxrc diff file is here: ftp://belyi.ac93.org/pub/KNOPPIX/linuxrc-knxapp.patch
And the source of a small helper I used: ftp://belyi.ac93.org/pub/KNOPPIX/knxtest.c

The way you create a new KNOPPIX image is simple: rebuild bootable ISO image without compressed KNOPPIX file and with the new minirt26.gz, append this KNOPPIX file to the image padded to 2K if necessary and this is the image you would burn to CD.

So, if you have current KNOPPIX in KNOPPIX-3.7.iso file, the procedure will be something like this:
# mount -o loop KNOPPIX-3.7.iso /mnt/src
# mkdir newimage
# rsync -a --exclude=KNOPPIX/KNOPPIX /mnt/src/ newimage
# cp minirt26-knxapp.gz newimage/boot/isolinux/minirt26.gz
# mkisofs -pad -l -r -J -v -V "KNOPPIX" -no-emul-boot -boot-load-size 4 -boot-info-table -b boot/isolinux/isolinux.bin -c boot/isolinux/boot.cat -hide-rr-moved -o KNOPPIX-prefix.iso newimage
# cat KNOPPIX-prefix.iso /mnt/src/KNOPPIX/KNOPPIX > KNOPPIX-new.iso
# dd if=/dev/zero bs=1 count=`ls -l KNOPPIX-new.iso | awk '{pad=$5%2048; if(pad) pad=2048-pad; print pad;}'` >> KNOPPIX-new.iso

The last step is necessary only if your CD burning software complains about the image not aligned to 2K block.
I wasn't able to come up with a procedure simpler than that.

Hope you like it.
Cheers,
Igor