It's not possible to use OverlayFS (mount -t overlay) to stack directories on a ZFS volume

---------- Configuration
# echo -e $(grep VERSION= /etc/os-release)\\nSIGNATURE=\"$(cat /proc/version_signature)\"
VERSION="16.04.3 LTS (Xenial Xerus)"
SIGNATURE="Ubuntu 4.10.0-35.39~16.04.1-generic 4.10.17"
# dpkg --list | grep zfs
ii libzfs2linux 0.6.5.6-0ubuntu18
ii zfs-doc 0.6.5.6-0ubuntu18
ii zfs-initramfs 0.6.5.6-0ubuntu18
ii zfs-zed 0.6.5.6-0ubuntu18
ii zfsutils-linux 0.6.5.6-0ubuntu18

---------- Fault: Creating an overlay of multiple directories on a ZFS volume does not work
# df /var/tmp
Filesystem Type 1K-blocks Used Available Use% Mounted on
tank07/var/tmp zfs 129916288 128 129916160 1% /var/tmp
# mkdir /var/tmp/{lower,middle,upper,workdir,overlay}
# mount -t overlay overlay -olowerdir=/var/tmp/middle:/var/tmp/lower,upperdir=/var/tmp/upper,workdir=/var/tmp/workdir /var/tmp/overlay
mount: wrong fs type, bad option, bad superblock on overlay,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
# dmesg|tail -1
[276328.438284] overlayfs: filesystem on '/var/tmp/upper' not supported as upperdir

---------- Control test 1: Creating an overlay of multiple directories on another filesystem works
# df /tmp
Filesystem Type 1K-blocks Used Available Use% Mounted on
tmpfs tmpfs 1048576 133492 915084 13% /tmp
# mkdir /tmp/{lower,middle,upper,workdir,overlay}
# mount -t overlay overlay -olowerdir=/tmp/middle:/tmp/lower,upperdir=/tmp/upper,workdir=/tmp/workdir /tmp/overlay
# mount | grep overlay
overlay on /tmp/overlay type overlay (rw,relatime,lowerdir=/tmp/middle:/tmp/lower,upperdir=/tmp/upper,workdir=/tmp/workdir)

---------- Control test 2: Creating an overlay using AuFS works on ZFS volume and elsewhere
# mount -t aufs -obr=/tmp/lower:/tmp/middle:/tmp/upper:/tmp/workdir none /tmp/overlay
# mount -t aufs -obr=/var/tmp/lower:/var/tmp/middle:/var/tmp/upper:/var/tmp/workdir none /var/tmp/overlay
# mount | grep aufs
none on /var/tmp/overlay type aufs (rw,relatime,si=9ead1ecae778b250)
none on /tmp/overlay type aufs (rw,relatime,si=9ead1ec9257d1250)

---------- Remark
While the use of AuFS can be used as a workaround in the above scenario, AuFS in turn will not work with [fuse.]glusterfs mounts (this has been documented elsewhere). Given that OverlayFS is part of the (upstream) kernel and Ubuntu now officially supports ZFS, the above should be fixed.
---
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Sep 18 16:12 seq
 crw-rw---- 1 root audio 116, 33 Sep 18 16:12 timer
AplayDevices: Error: [Errno 2] No such file or directory
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
DistroRelease: Ubuntu 16.04
IwConfig: Error: [Errno 2] No such file or directory
Lspci: Error: [Errno 2] No such file or directory
Lsusb: Error: [Errno 2] No such file or directory
MachineType: Red Hat KVM
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
Package: linux (not installed)
PciMultimedia:

ProcFB: 0 cirrusdrmfb
ProcKernelCmdLine: BOOT_IMAGE=/ROOT/ubuntu1604@/boot/vmlinuz-4.10.0-35-generic root=ZFS=tank07/ROOT/ubuntu1604 ro
ProcVersionSignature: Ubuntu 4.10.0-35.39~16.04.1-generic 4.10.17
RelatedPackageVersions:
 linux-restricted-modules-4.10.0-35-generic N/A
 linux-backports-modules-4.10.0-35-generic N/A
 linux-firmware 1.157.12
RfKill: Error: [Errno 2] No such file or directory
Tags: xenial
Uname: Linux 4.10.0-35-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups:

_MarkForUpload: True
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: 1.9.1-5.el7_3.3
dmi.chassis.type: 1
dmi.chassis.vendor: Red Hat
dmi.chassis.version: RHEL 7.3.0 PC (i440FX + PIIX, 1996)
dmi.modalias: dmi:bvnSeaBIOS:bvr1.9.1-5.el7_3.3:bd04/01/2014:svnRedHat:pnKVM:pvrRHEL7.3.0PC(i440FX+PIIX,1996):cvnRedHat:ct1:cvrRHEL7.3.0PC(i440FX+PIIX,1996):
dmi.product.name: KVM
dmi.product.version: RHEL 7.3.0 PC (i440FX + PIIX, 1996)
dmi.sys.vendor: Red Hat
---
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Sep 18 16:12 seq
 crw-rw---- 1 root audio 116, 33 Sep 18 16:12 timer
AplayDevices: Error: [Errno 2] No such file or directory
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
DistroRelease: Ubuntu 16.04
IwConfig: Error: [Errno 2] No such file or directory
Lspci: Error: [Errno 2] No such file or directory
Lsusb: Error: [Errno 2] No such file or directory
MachineType: Red Hat KVM
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
Package: linux (not installed)
PciMultimedia:

ProcFB: 0 cirrusdrmfb
ProcKernelCmdLine: BOOT_IMAGE=/ROOT/ubuntu1604@/boot/vmlinuz-4.10.0-35-generic root=ZFS=tank07/ROOT/ubuntu1604 ro
ProcVersionSignature: Ubuntu 4.10.0-35.39~16.04.1-generic 4.10.17
RelatedPackageVersions:
 linux-restricted-modules-4.10.0-35-generic N/A
 linux-backports-modules-4.10.0-35-generic N/A
 linux-firmware 1.157.12
RfKill: Error: [Errno 2] No such file or directory
Tags: xenial
Uname: Linux 4.10.0-35-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups:

_MarkForUpload: True
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: 1.9.1-5.el7_3.3
dmi.chassis.type: 1
dmi.chassis.vendor: Red Hat
dmi.chassis.version: RHEL 7.3.0 PC (i440FX + PIIX, 1996)
dmi.modalias: dmi:bvnSeaBIOS:bvr1.9.1-5.el7_3.3:bd04/01/2014:svnRedHat:pnKVM:pvrRHEL7.3.0PC(i440FX+PIIX,1996):cvnRedHat:ct1:cvrRHEL7.3.0PC(i440FX+PIIX,1996):
dmi.product.name: KVM
dmi.product.version: RHEL 7.3.0 PC (i440FX + PIIX, 1996)
dmi.sys.vendor: Red Hat