Proof-of-concept of approach #1

proof-of-concept.sh

#!/bin/sh
# This is a proof of concept.  It was not designed to be beautiful, portable or
# anything like that.
readonly SOURCE="${1}"
if [ -z "${SOURCE}" ]; then
  echo "Usage: ${0} <iso image> [/dev/sdX|vm name]"
  exit 1
fi
DEVICE=""
# Name of the virtual machine used for manual testing.
VMNAME="Fedora Live WS"
if [ -n "${2}" ]; then
  if echo "${2}" | grep -q "^/"; then
    readonly DEVICE="${2}"
    echo "Writing ${SOURCE} to ${DEVICE}"
  else
    VMNAME="${2}"
  fi
fi
readonly VMNAME

readonly MOUNTPOINT="$(mktemp -d)"
readonly MOUNTPOINT2="$(mktemp -d)"
# Extract iso.
mkdir -p "${MOUNTPOINT}"
sudo mount -o loop "${SOURCE}" "${MOUNTPOINT}"
readonly DIRECTORY="$(cat "${MOUNTPOINT}/EFI/BOOT/grub.cfg" | grep search | cut -d "'" -f2)"
mkdir -p "${DIRECTORY}"
rsync -av --delete "${MOUNTPOINT}/" "${DIRECTORY}"
sudo umount "${MOUNTPOINT}"
cd "${DIRECTORY}"
sudo mount -o loop ./images/efiboot.img "${MOUNTPOINT}"
sudo mount -o loop ./images/macboot.img "${MOUNTPOINT2}"
sudo sed -i \
  "s#rd.live.image#& rd.live.overlay=LABEL=OVERLAY:/OVERLAY.IMG#g" \
  ./isolinux/isolinux.cfg ./EFI/BOOT/grub.cfg ./isolinux/grub.conf \
  "${MOUNTPOINT}/EFI/BOOT/grub.cfg" "${MOUNTPOINT2}/EFI/BOOT/grub.cfg" \
  "${MOUNTPOINT2}/System/Library/CoreServices/grub.cfg"
sudo umount "${MOUNTPOINT}"
sudo umount "${MOUNTPOINT2}"
cd -

readonly LABEL="${DIRECTORY}"
readonly TARGET="${LABEL}.iso"
rm "${TARGET}"
mkisofs \
  -eltorito-boot "isolinux/isolinux.bin" \
  -eltorito-catalog "isolinux/boot.cat" \
  -input-charset utf-8 \
  -boot-load-size 4 \
  -boot-info-table \
  -no-emul-boot \
  -eltorito-alt-boot -efi-boot "images/efiboot.img" -no-emul-boot \
  -eltorito-alt-boot -efi-boot "images/macboot.img" -no-emul-boot \
  -rock -joliet \
  -volid "${LABEL}" \
  -translation-table \
  -graft-points \
  -output "${TARGET}" "${DIRECTORY}"
isohybrid --uefi --mac "${TARGET}"
implantisomd5 "${TARGET}"

IMAGE=""
if [ -z "${DEVICE}" ]; then
  IMAGE="${LABEL}.img"
  DEVICE="${IMAGE}"
fi
readonly IMAGE
if [ -n "${IMAGE}" ]; then
  echo "Creating ${IMAGE}.."
  dd "if=${TARGET}" "of=${IMAGE}" "bs=4M"
  # Make sure the fake drive is larger than the iso.
  fallocate -l 4000M "${IMAGE}"
fi

# Add fat partition.
echo -e "n\np\n\n\n\nt\n\nb\nw\n" | sudo fdisk "${DEVICE}"
if [ -n "${IMAGE}" ]; then
  readonly DEVICE="$(sudo losetup --partscan --show -f "${IMAGE}")p"
fi
sudo mkfs.fat -i DEAFBEEF -n OVERLAY "${DEVICE}4"
sudo mount "${DEVICE}4" "${MOUNTPOINT}"

SIZE="$(df | grep "${DEVICE}4" | sed -r "s/[^ ]+ +([0-9]+).+/\1/g")"
# Maximum size is 4 gb because of the fat32 limitation.
if [ "${SIZE}" -gt "$((4095 * 1024))" ]; then
  SIZE="$((4095 * 1024))"
fi
SIZE="$((${SIZE} / 4))"
readonly SIZE

echo "Creating overlay file ($((${SIZE} * 4)) kB).."
sudo dd "if=/dev/zero" "of=${MOUNTPOINT}/OVERLAY.IMG" "oflag=direct" \
  "count=${SIZE}" "bs=4K"
sudo umount "${MOUNTPOINT}"
if [ -n "${IMAGE}" ]; then
  sudo losetup -d "$(echo "${DEVICE}" | sed "s/.$//")"
  implantisomd5 --force "${IMAGE}"

  # For manual testing in VirtualBox.
  readonly VDISK="${LABEL}.vdi"
  rm -f "${VDISK}"
  VBoxManage convertfromraw --format VDI "${IMAGE}" "${VDISK}"

  # Reattach VDI.  It seems like VirtualBox can't boot from usb that's why it's
  # connected over sata.
  VBoxManage storageattach "${VMNAME}" --storagectl SATA --port 0 --medium none
  VBoxManage closemedium disk "${VDISK}"
  VBoxManage storageattach "${VMNAME}" --storagectl SATA --port 0 --type hdd \
    --medium "${VDISK}"
else
  sudo implantisomd5 --force "${DEVICE}"
fi