Put this script at /sbin/mount.ovtmp . Script for tmpfs overlays. Put a line for mounting an "ovtmp" fs into fstab, and it'll use a tmpfs and overlayfs to mount over the target. Useful if the root is readonly, but changes need to be possible but don't need persistance.

mount.ovtmp

#!/bin/sh

set -e

cleanup(){
  set +e
  if [ -n "$tmp" ]
  then
    umount -l "$tmp" 2>/dev/null
    rmdir "$tmp"
  fi
}

die(){
  echo "Usage: $0 name target" >&2
  exit 1
}

dir=
name=
options=

i=0
while [ $# -gt 1 ]
do
  arg="$1"
  shift
  case "$arg" in
    -o*)
      if [ "$arg" = -o ]
        then arg="$1"; shift
        else arg="${arg#-o}"
      fi
      options="$arg"
    ;;
    -*) die ;;
    *)
      i=$(expr $i + 1)
      case "$i" in
        1) name="$arg" ;;
        2) dir="$arg" ;;
        *) die ;;
      esac
    ;;
  esac
done
arg=

if [ -z "$name" ]
  then name="ovtmp"
fi

if [ -z "$dir" ]
then
  echo "Usage: $0 name target"
  exit 1
fi

trap cleanup exit

tmp=$(mktemp -d /tmp/.ovtmp.XXXXXX)
mount -t tmpfs -o "$options" "$name.tmpfs" "$tmp"
mkdir -p "$tmp/u" "$tmp/w"
mount -t overlay "$name" -o lowerdir="$dir",upperdir="$tmp/u",workdir="$tmp/w" "$dir"