Execute one-time script after unattended Kali install

I've been able to build an ISO for an unattended install. It works great, adding the latest packages, partitioning, etc. What I just can't get to work is adding a one-time script to run once after the install is over. It will comment out deb-src, do some VM changes, etc. The d-i late_command string in the preseed just doesn't work. Here's what I've tried:

1. Putting a "post_install.sh" file in kali-config/common/includes.installer/ and putting "d-i preseed/late_command string in-target /bin/bash /target/install/post_install.sh" at the end of the preseed.cfg.
2. Putting a "post_install.sh" file in kali-config/common/includes.chroot/ and running "d-i preseed/late_command string in-target /bin/bash /root/post_install.sh;" at the end of the preseed.cfg.
3. Running "d-i preseed/run string post_install.sh" at the end of the preseed.cfg.
4. Variations of these.

Nothing worked. So my questions are:

1. What is the format to run a one-time-use script to modify the created filesystem just before completing an unattended install? And
2. In which folder of the live-build setup should this script be placed?
2a. Secondarily, can this be done using the live build hooks provided by live-build-config for Kali?