you shouldn't have a problem adding "splash" to the default boot sequence since I've done this many times. I've used custom isolinux.cfg changes so friends can run Knoppix on their systems without having to enter cheatcodes yet it'll boot correctly for their custom setup. For instance, I'd specify fromhd=/dev/hda1 myconfig=/dev/hda1 home=/dev/hda2 and splash as added cheatcodes in isolinux.cfg. This brings up a point which might work out for you and your clients setup.

If your setup allows you to run your clients custom code from a RO compressed filesystem( ie from a CDROM ) then an easy install is to create the /boot partition, on the harddrive, with the stuff from /boot/isolinux, copy /KNOPPIX/* from the CD to the HD( like tohd= does ), create any HD directory structures needed for application data storage, and then run lilo or grub to set /boot as the boot system. Putting in the correct isolinux.cfg file for the HD boot so subsequent HD booting would then bring the system up ready for your customer. It'll help protect the runtime filesystem from corruption and make upgrading a matter of just updating one file on the HD( /KNOPPIX/KNOPPIX or /CUSTOM/CLIENT-NAME in your case ) and only updating the /boot files if you change more than your clients custom code in the compressed image file.

I've done pretty much all of this already, with the exception of creating the boot area on the HD. So far, I've just used a slight customization of the knoppix-terminalserver process so that remote servers get the boot loader from one remote server and from then on, they run autonomously without the boot server since they are using a local HD filesystem image instead of the remote systems /cdrom image. I've also used customized knoppix.sh files to chain load a custom runtime script for tweaking things like the /etc/hosts, /etc/apache/http.conf, /etc/samba/smb*, etc files. It sure beats doing a full HD install and is alot quicker too.

Hopefully this info was helpful and I'm looking forward to hearing how your system comes together.