@ Werner P. Schulz
... simple and nice solution ...
Indeed! If rspiteri's client is using Win7 with separated boot and system partitions, his client won't even realise the knoppix-data.img has been planted in the /dev/sda1 since the boot partition won't be shown in the Windows Explorer. Cheeky, cheeky...

@ rajibando
(2) Booted to the laptop owner's system normally.
(3) Copied your pendrive's boot and KNOPPIX folders, renamed your pendrive's KNOPPIX binary to KNOPPIX000.
(4) Booted from your pendrive, let your syslinux search and find the HDD's KNOPPIX binary and knoppix-data.inf and boot from there.
If (big if) I am not mistaken, the internal hard discs generally occur earlier than the usb storage devices on the Linux's device list, and KNOPPIX's init will use the first KNOPPIX/KNOPPIX found in the system by default. So there may be no need to re-name KNOPPIX to KNOPPIX000 as mentioned in point 3. ... NAND flash memory's wear leveling... Ouch...