Tested functional for what I'm after, adding zfs to the boot-initrd.

$ bzr init-repo maas-images
$ cd maas-images
$ bzr branch lp:maas-images trunk
$ bzr branch lp:~smoser/maas-images/trunk.add-zfs add-zfs

$ rel=xenial ; arch=amd64
$ burl=http://cloud-images.ubuntu.com/
$ file=$PWD/$rel-server-cloudimg-amd64.squashfs
$ wget $burl/$rel/current/$rel-server-cloudimg-amd64.squashfs -O "$file"

$ for d in add-zfs trunk; do
   ( cd $d &&
     PATH=$PWD/bin:$PATH ./bin/meph2-build --config=conf/meph-v3.yaml \
        $arch $rel $(date +"%Y%m%d") $file $rel-output.d/ 2>&1 ) |
   tee $d-out.log
  done

$ for initrd in $(cd trunk && find $rel-output.d -name boot-initrd ); do
    for br in add-zfs trunk; do
       [ -f "$br/$initrd.list" ] && continue
       lsinitramfs $br/$initrd | grep -v "boot-initrd" | sort > $br/$initrd.list
    done
    echo ==== $initrd ====
    diff -u trunk/$initrd.list add-zfs/$initrd.list
done

==== xenial-output.d/xenial/amd64/20171213/hwe-16.04/lowlatency/boot-initrd ====
--- trunk/xenial-output.d/xenial/amd64/20171213/hwe-16.04/lowlatency/boot-initrd.list 2017-12-13 19:52:30.218963551 +0000
+++ add-zfs/xenial-output.d/xenial/amd64/20171213/hwe-16.04/lowlatency/boot-initrd.list 2017-12-13 19:52:24.419032666 +0000
@@ -2006,6 +2006,19 @@
 lib/modules/4.10.0-42-lowlatency/kernel/virt
 lib/modules/4.10.0-42-lowlatency/kernel/virt/lib
 lib/modules/4.10.0-42-lowlatency/kernel/virt/lib/irqbypass.ko
+lib/modules/4.10.0-42-lowlatency/kernel/zfs
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/avl
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/avl/zavl.ko
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/nvpair
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/nvpair/znvpair.ko
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/spl
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/spl/spl.ko
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/unicode
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/unicode/zunicode.ko
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/zcommon
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/zcommon/zcommon.ko
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/zfs
+lib/modules/4.10.0-42-lowlatency/kernel/zfs/zfs/zfs.ko
 lib/modules/4.10.0-42-lowlatency/modules.alias
 lib/modules/4.10.0-42-lowlatency/modules.alias.bin
 lib/modules/4.10.0-42-lowlatency/modules.builtin
==== xenial-output.d/xenial/amd64/20171213/hwe-16.04/generic/boot-initrd ====
--- trunk/xenial-output.d/xenial/amd64/20171213/hwe-16.04/generic/boot-initrd.list 2017-12-13 19:52:41.686828209 +0000
+++ add-zfs/xenial-output.d/xenial/amd64/20171213/hwe-16.04/generic/boot-initrd.list 2017-12-13 19:52:36.094894204 +0000
@@ -2006,6 +2006,19 @@
 lib/modules/4.10.0-42-generic/kernel/virt
 lib/modules/4.10.0-42-generic/kernel/virt/lib
 lib/modules/4.10.0-42-generic/kernel/virt/lib/irqbypass.ko
+lib/modules/4.10.0-42-generic/kernel/zfs
+lib/modules/4.10.0-42-generic/kernel/zfs/avl
+lib/modules/4.10.0-42-generic/kernel/zfs/avl/zavl.ko
+lib/modules/4.10.0-42-generic/kernel/zfs/nvpair
+lib/modules/4.10.0-42-generic/kernel/zfs/nvpair/znvpair.ko
+lib/modules/4.10.0-42-generic/kernel/zfs/spl
+lib/modules/4.10.0-42-generic/kernel/zfs/spl/spl.ko
+lib/modules/4.10.0-42-generic/kernel/zfs/unicode
+lib/modules/4.10.0-42-generic/kernel/zfs/unicode/zunicode.ko
+lib/modules/4.10.0-42-generic/kernel/zfs/zcommon
+lib/modules/4.10.0-42-generic/kernel/zfs/zcommon/zcommon.ko
+lib/modules/4.10.0-42-generic/kernel/zfs/zfs
+lib/modules/4.10.0-42-generic/kernel/zfs/zfs/zfs.ko
 lib/modules/4.10.0-42-generic/modules.alias
 lib/modules/4.10.0-42-generic/modules.alias.bin
 lib/modules/4.10.0-42-generic/modules.builtin
==== xenial-output.d/xenial/amd64/20171213/hwe-16.04-edge/lowlatency/boot-initrd ====
--- trunk/xenial-output.d/xenial/amd64/20171213/hwe-16.04-edge/lowlatency/boot-initrd.list 2017-12-13 19:52:55.338667092 +0000
+++ add-zfs/xenial-output.d/xenial/amd64/20171213/hwe-16.04-edge/lowlatency/boot-initrd.list 2017-12-13 19:52:48.490747910 +0000
@@ -2224,6 +2224,19 @@
 lib/modules/4.13.0-19-lowlatency/kernel/virt
 lib/modules/4.13.0-19-lowlatency/kernel/virt/lib
 lib/modules/4.13.0-19-lowlatency/kernel/virt/lib/irqbypass.ko
+lib/modules/4.13.0-19-lowlatency/kernel/zfs
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/avl
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/avl/zavl.ko
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/nvpair
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/nvpair/znvpair.ko
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/spl
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/spl/spl.ko
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/unicode
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/unicode/zunicode.ko
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/zcommon
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/zcommon/zcommon.ko
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/zfs
+lib/modules/4.13.0-19-lowlatency/kernel/zfs/zfs/zfs.ko
 lib/modules/4.13.0-19-lowlatency/modules.alias
 lib/modules/4.13.0-19-lowlatency/modules.alias.bin
 lib/modules/4.13.0-19-lowlatency/modules.builtin
==== xenial-output.d/xenial/amd64/20171213/hwe-16.04-edge/generic/boot-initrd ====
--- trunk/xenial-output.d/xenial/amd64/20171213/hwe-16.04-edge/generic/boot-initrd.list 2017-12-13 19:53:09.354502437 +0000
+++ add-zfs/xenial-output.d/xenial/amd64/20171213/hwe-16.04-edge/generic/boot-initrd.list 2017-12-13 19:53:02.442583253 +0000
@@ -2224,6 +2224,19 @@
 lib/modules/4.13.0-19-generic/kernel/virt
 lib/modules/4.13.0-19-generic/kernel/virt/lib
 lib/modules/4.13.0-19-generic/kernel/virt/lib/irqbypass.ko
+lib/modules/4.13.0-19-generic/kernel/zfs
+lib/modules/4.13.0-19-generic/kernel/zfs/avl
+lib/modules/4.13.0-19-generic/kernel/zfs/avl/zavl.ko
+lib/modules/4.13.0-19-generic/kernel/zfs/nvpair
+lib/modules/4.13.0-19-generic/kernel/zfs/nvpair/znvpair.ko
+lib/modules/4.13.0-19-generic/kernel/zfs/spl
+lib/modules/4.13.0-19-generic/kernel/zfs/spl/spl.ko
+lib/modules/4.13.0-19-generic/kernel/zfs/unicode
+lib/modules/4.13.0-19-generic/kernel/zfs/unicode/zunicode.ko
+lib/modules/4.13.0-19-generic/kernel/zfs/zcommon
+lib/modules/4.13.0-19-generic/kernel/zfs/zcommon/zcommon.ko
+lib/modules/4.13.0-19-generic/kernel/zfs/zfs
+lib/modules/4.13.0-19-generic/kernel/zfs/zfs/zfs.ko
 lib/modules/4.13.0-19-generic/modules.alias
 lib/modules/4.13.0-19-generic/modules.alias.bin
 lib/modules/4.13.0-19-generic/modules.builtin
==== xenial-output.d/xenial/amd64/20171213/ga-16.04/lowlatency/boot-initrd ====
--- trunk/xenial-output.d/xenial/amd64/20171213/ga-16.04/lowlatency/boot-initrd.list 2017-12-13 19:53:20.150376312 +0000
+++ add-zfs/xenial-output.d/xenial/amd64/20171213/ga-16.04/lowlatency/boot-initrd.list 2017-12-13 19:53:14.806438744 +0000
@@ -1749,6 +1749,19 @@
 lib/modules/4.4.0-103-lowlatency/kernel/ubuntu/bnxt/bnxt_en_bpo.ko
 lib/modules/4.4.0-103-lowlatency/kernel/ubuntu/i915
 lib/modules/4.4.0-103-lowlatency/kernel/ubuntu/i915/i915_bpo.ko
+lib/modules/4.4.0-103-lowlatency/kernel/zfs
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/avl
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/avl/zavl.ko
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/nvpair
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/nvpair/znvpair.ko
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/spl
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/spl/spl.ko
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/unicode
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/unicode/zunicode.ko
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/zcommon
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/zcommon/zcommon.ko
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/zfs
+lib/modules/4.4.0-103-lowlatency/kernel/zfs/zfs/zfs.ko
 lib/modules/4.4.0-103-lowlatency/modules.alias
 lib/modules/4.4.0-103-lowlatency/modules.alias.bin
 lib/modules/4.4.0-103-lowlatency/modules.builtin
==== xenial-output.d/xenial/amd64/20171213/ga-16.04/generic/boot-initrd ====
--- trunk/xenial-output.d/xenial/amd64/20171213/ga-16.04/generic/boot-initrd.list 2017-12-13 19:53:31.858239533 +0000
+++ add-zfs/xenial-output.d/xenial/amd64/20171213/ga-16.04/generic/boot-initrd.list 2017-12-13 19:53:25.686311638 +0000
@@ -1751,6 +1751,19 @@
 lib/modules/4.4.0-103-generic/kernel/ubuntu/bnxt/bnxt_en_bpo.ko
 lib/modules/4.4.0-103-generic/kernel/ubuntu/i915
 lib/modules/4.4.0-103-generic/kernel/ubuntu/i915/i915_bpo.ko
+lib/modules/4.4.0-103-generic/kernel/zfs
+lib/modules/4.4.0-103-generic/kernel/zfs/avl
+lib/modules/4.4.0-103-generic/kernel/zfs/avl/zavl.ko
+lib/modules/4.4.0-103-generic/kernel/zfs/nvpair
+lib/modules/4.4.0-103-generic/kernel/zfs/nvpair/znvpair.ko
+lib/modules/4.4.0-103-generic/kernel/zfs/spl
+lib/modules/4.4.0-103-generic/kernel/zfs/spl/spl.ko
+lib/modules/4.4.0-103-generic/kernel/zfs/unicode
+lib/modules/4.4.0-103-generic/kernel/zfs/unicode/zunicode.ko
+lib/modules/4.4.0-103-generic/kernel/zfs/zcommon
+lib/modules/4.4.0-103-generic/kernel/zfs/zcommon/zcommon.ko
+lib/modules/4.4.0-103-generic/kernel/zfs/zfs
+lib/modules/4.4.0-103-generic/kernel/zfs/zfs/zfs.ko
 lib/modules/4.4.0-103-generic/modules.alias
 lib/modules/4.4.0-103-generic/modules.alias.bin
 lib/modules/4.4.0-103-generic/modules.builtin