Applying Mac firmware updates manually Recently an important change regarding Mac firmware installation was introduced.
Apple does no longer provide up-to-date standalone firmware updaters.
(Older firmwares are still available.  (Note the addition of: &quot;This article has been archived and is no longer updated by Apple.&quot;)
Instead one has to heed this advice:
Upgrade macOS on a Mac at your institution:

If you're the system administrator for your business or education institution, use these methods to upgrade macOS High Sierra.
Apple doesn't recommend or support monolithic system imaging when upgrading or updating macOS.
Monolithic system imaging can only be used to re-install macOS, not to upgrade to a new macOS version.
If you try to use a monolithic system image, required firmware updates will be missing from the installation. This causes the Mac to operate in an unsupported and unstable state. You can use system images to re-install the existing operating system on a Mac.

That seems to mean that the answer given here:
Is it possible to reinstall MacBook firmware? might no longer be valid for those firmware upgrades (needed for APFS) in macOS High Sierra 10.13 coming within the installer for High Sierra and that they are only applied during the installation process.
How can these firmware updates extracted from the installer or the system and applied manually? Are there other triggers to apply the latest firmware on affected Macs?
It is necessary to obtain the installer application for macOS High Sierra. Once that is available in /Applications one can use the following script to extract the firmware data and install scripts to produce a standalone firmware installer package:

#!/bin/sh
# Based on investigations and work by Pepijn Bruienne
# Expects a single /Applications/Install macOS High Sierra*.app on disk    
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

IDENTIFIER="com.foo.FirmwareUpdateStandalone"
VERSION=1.0

# find the Install macOS High Sierra.app and mount the embedded InstallESD disk image
echo "Mounting High Sierra ESD disk image..."
/usr/bin/hdiutil mount /Applications/Install\ macOS\ High\ Sierra*.app/Contents/SharedSupport/InstallESD.dmg

# expand the FirmwareUpdate.pkg so we can copy resources from it
echo "Expanding FirmwareUpdate.pkg"
/usr/sbin/pkgutil --expand /Volumes/InstallESD/Packages/FirmwareUpdate.pkg /tmp/FirmwareUpdate

# we don't need the disk image any more
echo "Ejecting disk image..."
/usr/bin/hdiutil eject /Volumes/InstallESD

# make a place to stage our pkg resources
/bin/mkdir -p /tmp/FirmwareUpdateStandalone/scripts

# copy the needed resources
echo "Copying package resources..."
/bin/cp /tmp/FirmwareUpdate/Scripts/postinstall_actions/update /tmp/FirmwareUpdateStandalone/scripts/postinstall
# add an exit 0 at the end of the script
echo "" &gt;&gt; /tmp/FirmwareUpdateStandalone/scripts/postinstall
echo "" &gt;&gt; /tmp/FirmwareUpdateStandalone/scripts/postinstall
echo "exit 0" &gt;&gt; /tmp/FirmwareUpdateStandalone/scripts/postinstall
/bin/cp -R /tmp/FirmwareUpdate/Scripts/Tools /tmp/FirmwareUpdateStandalone/scripts/

# build the package
echo "Building standalone package..."
/usr/bin/pkgbuild --nopayload --scripts /tmp/FirmwareUpdateStandalone/scripts --identifier "$IDENTIFIER" --version "$VERSION" /tmp/FirmwareUpdateStandalone/FirmwareUpdateStandalone.pkg

# clean up
/bin/rm -r /tmp/FirmwareUpdate
/bin/rm -r /tmp/FirmwareUpdateStandalone/scripts


This is part of the Scripts of possible interest to macOS admins
from the munki project.