How do I make a cheat-code permanent on Knoppix 8.6.1?

Summary:

How do I make the following cheat-code permanent on knoppix 8.6.1 on usb stick,
because filesystem type is iso9660 (read-only) and won't let me edit
/mnt-system/boot/isolinux/isolinux.cfg.

Cheat code entered at boot prompt: knoppix xmodule=vesa


Full Details:

I've just put knoppix 8.6.1 on a usb stick (persistence was there by default).
But X Window won't display unless I enter the cheat-code "knoppix xmodule=vesa"
at the initial boot prompt.

Incidentally, when using Knoppix 7.7.1 on a usb stick, on the EXACT SAME laptop
(Acer Aspire 5755G), I don't need the cheat-code.

I'd like to not have to type the cheat-code in every time I boot the 8.6.1, and
believe that I can achieve that by adding "xmodule=vesa" as a kernel parameter
on an "APPEND" line in /boot/isolinux/isolinux.cfg.

I can get to that file using the path
/mnt-system/boot/isolinux/isolinux.cfg.

But it won't let me write to that file because /mnt-system is mounted read
only. When I try to remount /mnt-system as read-write, it says "write-protected."

/UNIONFS is mounted as read-write, but the path /UNIONFS/boot/isolinux/ does not
exist.

I understand that because partition 1 (the main Knoppix system) is filesystem
type iso9660*, that it is by definition read-only. (*On which I believe
/boot/isolinux/isolinux.cfg sits).

Main Question:

So could you tell me please, what method I need to use to make my cheat-code
permanent?

Reason for Using 8.6.1:

I'm trying out 8.6.1 because it has a newer kernel than 7.7.1, and I've read
that a newer kernel might fix my problem of downloads halting when the lid is
closed. I've already tried many lid disabling methods, but none of them prevent
the downloading from halting.

I do not understand fully your USB stick situation: has it more partitions, 1 containing the Knoppix system with ISO9660 fs (readonly, optical CD fs), and an other one that is rw (maybe FAT32) for persistence?
If so, you cannot change the ISO9660 fs since it is like this: readonly.
I recommend you to reformat the stick with GParted (for example) to FAT32 fs, and only 1 partition. Then install GRUB2 bootloader to stick (you can use SystemRescueCD). Make a boot directory in the root of the created partition. Store all your live systems here in "boot". Depending of the size of USB stick you can download and store many systems.
Download Knoppix ISO. If you have enough memory, download in /tmp directory (in memory). Extract its whole content into boot/knx/KNOPPIX_NAME directory.
Then create grub/grub.cfg file and enter in it the appropriate menuentries for Knoppix.
In the same way you can download and start other live sytems with GRUB2 from the USB stick. You can use persistence, too.
On the net you will find a lot of information about GRUB2, grub.cfg and menuentry examples for different systems. Also for Knoppix...
If you take my advice and boot Knoppix in this way, I can give you (if you need) working menuentries for Knoppix, since I use it just in this way: started with GRUB2 from internal and external drives, USB sticks.

Hello, Winger9, kindly accept my wellwishes.
Have you searched exhaustively the forum threads?