I have managed to change the myconfig=scan to a bootable format using the isolinx.conf changes in the boot directory.

This requires a re-burn through the mkisofs routine.

My system with the myconfig=scan included in the isolinux.conf, which does require a single keybd character entry at boot prompt, permits the myconfig=scan option along with any other desired option( in my case.... toram).

The procedure I utilized did not seem to be sensitive to the size of the .iso. This would seem reasonable since the size of the boot parameters entered at boot time is not restricted...can be quite a number of characters.

It would seem possible to enter the character I use to select boot mode..... automatically at boot prompt .

Perhaps that is what you seek.