Alright, ever since tinybit's 1st post more than 10 years ago,
Ubuntu LiveCD.iso successfully booted from USB thumbdrive:
title Ubuntu LiveCD find --set-root /ubuntu-9.04-beta-desktop-i386.iso map /ubuntu-9.04-beta-desktop-i386.iso (0xff) map --hook root (0xff) kernel /casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper iso-scan/filename=/ubuntu-9.04-beta-desktop-i386.iso quiet splash locale=zh_CN.UTF-8 -- initrd /casper/initrd.gz boot
You may replace the Chinese locale "locale=zh_CN.UTF-8" with your own locale, or simply strip it out for the default English locale.
Ubuntu LiveCD.iso has been working, all the way to [Ubuntu 18.04 ISO](https://ubuntuforums...d.php?t=2398574), however I found it doesn't work for me.
I can see from the boot process, a printout indicating the ISO has been found, showing it is of type joite or something similar. Then an error message of file not found, and an url for the grub4dos home page. So I guess the error is still from within grub4dos. Not the iso itself.
Again, here is the entry that I used:
title Ubuntu 18
find --set-root /iso/ubt1804.iso
map --heads=0 --sectors-per-track=0 /iso/ubt1804.iso (0xff)
map --hook
root (0xff)
kernel /casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper iso-scan/filename=/iso/ubt1804.iso splash
initrd /casper/initrd.lz