I hacked the knx-hdinstall and /etc/fstab to do this.

I formatted and mounted my partitions manually onto /mnt/target, hacking out the size check, format and mount operations performed by the script.

Here are files as I changed them.

http://steveredlich.com/~steve/knx-hdinstall
http://steveredlich.com/~steve/knx-fstab

Steve