This script allows you to boot some ISO images from GRUB.

grub-iso.sh

#!/bin/sh

prefix=/usr
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
. ${libdir}/grub/grub-mkconfig_lib

ROOTDIR=""
ISODIR="home/shared/iso"

if [ "x${GRUB_DEVICE_UUID}" = "x" ] || [ "x${GRUB_DISABLE_LINUX_UUID}" = "xtrue" ] \
    || ! test -e "/dev/disk/by-uuid/${GRUB_DEVICE_UUID}" \
    || uses_abstraction "${GRUB_DEVICE}" lvm; then
  ROOT_DEVICE=${GRUB_DEVICE}
else
  ROOT_DEVICE=UUID=${GRUB_DEVICE_UUID}
fi

require_tmpdir() {
  if [ -z "$TMPDIR" ]; then
    if type mktemp >/dev/null 2>&1; then
      export TMPDIR=`mktemp -d /tmp/grub-iso.XXXXXX`
    else
      mkdir /tmp/grub-iso
      export TMPDIR=/tmp/grub-iso
    fi
  fi
}

add_entry_from_iso() {
	ISO_MOUNT="$1"
	ISO_NAME="$2"
	ISO_PATH=`make_system_path_relative_to_its_root "/$ISODIR/$ISO_NAME"`
	
	if [ -e "$ISO_MOUNT/.disk/info" ]; then
		TITLE="ISO image: "`cat "$ISO_MOUNT/.disk/info"`" ($ISO_NAME)"
	else
		TITLE="ISO image: "$ISO_NAME
	fi

	if [ -e "$ISO_MOUNT/boot/grub/loopback.cfg" ]; then
		echo "submenu '$TITLE' {"
		sed "s@menuentry.*{@&\nsearch --set -f \"$ISO_PATH\"\nloopback loop \"$ISO_PATH\"@;s@\${iso_path}@\"$ISO_PATH\"@;s@\(linux\|initrd\)\w*\s*@&(loop)@" "$ISO_MOUNT/boot/grub/loopback.cfg"
		echo "}"
	elif [ -d "$ISO_MOUNT/live" ]; then
		echo "submenu '$TITLE' {"
		echo "	menuentry 'Default settings' {"
		echo "		search --set -f \"$ISO_PATH\""
		echo "		loopback loop \"$ISO_PATH\""
		echo "		linux (loop)/live/vmlinuz"
		echo "		initrd (loop)/live/initrd.img boot=live config noswap noprompt nosplash"
		echo "	}"
		grep "([[:blank:]]*(menu label|kernel|append))" "$ISO_MOUNT/isolinux/live.cfg"
		# | sed "s@menu label\s*\(.*\)\s*\n@menuentry \"\1\" {\n@;s@kernel\s*\(.*\)\s*\n@linux (loop)\1\n@;s@append\s*initrd=\(.*\)\s*\(.*\)\n@initrd (loop)\1 \2@"
		echo "}"
	else
		return 1
	fi

	return 0
}

for f in "$ROOTDIR/$ISODIR"/*; do
	if [ ! -e "$f" ]; then
		continue
	fi
	name=${f##*/}
	require_tmpdir
	mount -o loop "$f" "$TMPDIR" > /dev/null 2>&1
	if add_entry_from_iso "$TMPDIR" "$name"; then
		echo "Found ISO image: $name" >&2
	fi
	umount -l "$TMPDIR" > /dev/null 2>&1
done

help.md

Usage

1. Put file grub.iso in /etc/grub.d and rename it according to the desired position in the boot menu (usually you want it to run somewhere at the end, so you should name it e. g. 90_grub-iso)
2. Change the ROOTDIR variable to the path to the root filesysytem, without the trailing slash (normally you should leave this empty) and ISODIR to the path to the directory in which you have the ISO files, without the leading and trailing slash (can be on any partition).
3. Run # update-grub. You should see lines like Found ISO image: ... if the ISO files were successfully added to the boot menu.