How to set up hostname for new servers from custom Image without using Stackscript?

I'm trying to combine custom image with custom StackScript but it doesn't look like the GUI (or API) allows this.

My use case? I'd like to create a baseline image with some initial common configuration (package addition & updates, user setup) - Image seems like the best solution. StackScripts apparently don't like "reboot" in them and whole initial setup (including reboot) can take a couple minutes, for every new server.

I could bunch it all up in an image, but there's one exception I've found - hostname setup. Since image is a baseline, setting up hostname in it does not make sense. I have the following simple stack script but I cannot attach it to custom-image-based servers:

#!/bin/bash
set -ev

#<UDF name="hostname" label="Linode hostname">
# HOSTNAME=
#<UDF name="fqdn" label="Linode FQDN">
# FQDN=
IPADDR=$(/sbin/ifconfig eth0 | awk '/inet / { print $2 }' | sed 's/addr://')

# hostname setup
echo $HOSTNAME > /etc/hostname
hostname -F /etc/hostname

echo "$IPADDR $FQDN $HOSTNAME" >> /etc/hosts

On the other hand putting it as a startup service (r.g. a systemd unit) seems like a bad move, I generally only want to run it once - during provisioning.