So, I wanted to remaster the SystemRescue iso. A trivial (20 character addition) was needed. I could not for the life of me figure out how to do this.
The answer (after 3-4 different multi-hour sessions trying to figure this out) was to use a tool called: xorriso. And to bear in mind that the iso itself needs to be bootable via an ISO or burnt CD/DVD as well as via UEFI and possible even via USB. This means it needs to be an everything bagel (so to speak). I found the "howto" for another distro and used that technique with the slight differences for the system rescue iso (which buries the efiboot.img under archiso since it is arch linux based.)
There was also this handy little guide re: xorriso. That was part of figuring this out. (I've always used genisoimage and friends prior to needing to remaster this.)