The following boot menu specifications seem to work.
…
menuentry 'GParted 0.32.0-1' --class Debian --class Linux {
insmod part_msdos
insmod ext2
ISO=/ISO/gparted-live-0.32.0-1-amd64.iso
search -f --set --no-floppy $ISO
loopback xyz $ISO
linux (xyz)/live/vmlinuz boot=live config union=overlay username=user noswap noprompt ip=frommedia findiso=/boot$ISO toram=filesystem.squashfs lang=de keyboard=de
initrd (xyz)/live/initrd.img
}
menuentry 'Grml 2017-05' --class Debian --class Linux {
insmod part_msdos
insmod ext2
ISO=/ISO/grml64-full_2017.05.iso
search -f --set --no-floppy $ISO
loopback xyz $ISO
linux (xyz)/boot/grml64full/vmlinuz boot=live live-media-path=/live/grml64-full/ findiso=/boot$ISO bootid=grml64full201705 grml2ram ssh=secret noeject noprompt lang=de vga=0x31b startx
initrd (xyz)/boot/grml64full/initrd.img
}
…
Do you get any further ideas for fixing another configuration approach?
…
menuentry 'Knoppix 8.2' --class Debian --class Linux {
insmod part_msdos
insmod ext2
ISO='/ISO/KNOPPIX_V8.2-2018-05-10-DE.iso'
search -f --set --no-floppy $ISO
loopback xyz $ISO
linux (xyz)/boot/isolinux/linux64 findiso=${ISO} ${GRUB_CMDLINE_LINUX_DEFAULT} ${GRUB_CMDLINE_LINUX} lang=de ${EXTRA}
initrd (xyz)/boot/isolinux/minirt.gz
}
…