Hi future me,
Sometimes you’re going to find yourself needing to boot some very archaic CDs. But CD drives might not exist in the future, so you’re stuck with USB to shim the ISO. (I think that first sentence should get all of the Google hits, but lets include some more buzzwords such as LiveCD, LiveUSB, syslinux etc.)
What you need is a syslinux USB drive (SD cards work too), and make use of the memdisk “kernel”, which really isn’t a kernel.
The idea is to boot syslinux from bios/mbr, then use memdisk (provided by syslinux in /usr/share/syslinux/memdisk or ./memdisk/memdisk from built source) to boot the ISO.
A better option would be to follow the Gentoo LiveUSB, but we can’t always be assured that the boot process will be that simple, e.g. DOS and it’s many variants.
The real bit
Assuming that syslinux is installed on D:\ (because this is a windows guide), with a pre-existing D:\syslinux.cfg (because you followed the Gentoo guide above).
Place memdisk and your iso file (as an iso file, no raw writing to disk here!) in D:\ too.
Add this entry to syslinux.cfg
label myisofile kernel memdisk initrd myisofile.iso append iso