Running Script after Preseed Install

Hello there,

If you don't want to here the long version, but still want to answer my question, please skip down to THE BOTTOM!

You can probably tell what I want to do already from the title, so I'll sum up what I have already done real quick:

First off, for those that don't know, a preseed file is a script which auto answers all the questions in the install process for you, so you can start it, walk away, and come back with a full system without having to babysit the install and answer all the questions. Here is a handy page if you are interested yourself:
https://help.ubuntu.com/7.04/installation-guide/powerpc/preseed-using.html

1) Tried installing from CD and specifying to look on a thumb drive for the preseed file through a boot parameter. Could not get it to find the file, not sure where it would look or how it would mount the dive.
2) Made a boot thumb drive, and placed preseed file on there. Worked fine, but didn't find file until it looked for it while looking for iso on the thumb drive.
3) Improved number of questions I had to answer by placing file right in the initrd file. This is tricky, if you are curious about it, you can ask. This should, in theory, also work if you edited the initrd on an iso and burned it.

Now, at the end of the install, I want to run a script that does some further set up aside from the installation. Here is a section of the default provided preseed file:
Code:

# d-i preseeding is inherently not secure. Nothing in the installer checks
# for attempts at buffer overflows or other exploits of the values of a
# preconfiguration file like this one. Only use preconfiguration files from
# trusted locations! To drive that home, and because it's generally useful,
# here's a way to run any shell command you'd like inside the installer,
# automatically.

# This first command is run as early as possible, just after
# preseeding is read.
#d-i preseed/early_command string anna-install some-udeb

# This command is run just before the install finishes, but when there is
# still a usable /target directory. You can chroot to /target and use it
# directly, or use the apt-install and in-target commands to easily install
# packages and run commands in the target system.
#d-i preseed/late_command string apt-install zsh; in-target chsh -s /bin/zsh

Now, using this new information, I want to run a script. Trouble is, that when I do this, using
d-i preseed/late_command string sh testscript.sh
I get an error that the file cannot be found. My theory is that during the final stages of the install, it is running from the hard drive and not the installation device. If this is wrong, or you have a different solution than the answer to the question at the bottom, please feel free to voice it.

// THE BOTTOM \\

Therefore, during installation, I want to try to place a file from the boot device onto the hard drive. How would I go about doing this, what/where on the .iso do I edit?

// Temporary solution \\

Copy the file from an in house mirror:

d-i preseed/late_command string \
in-target chsh -s /bin/bash ; \
in-target wget http://[some_IP]/testscript.sh ; \
in-target sh testscript.sh ;
in-target rm testscript.sh ;

My speculation would be that the device is okay but you need an explicit path to the script.

As a workaround, make it install a dummy package with a preinst script which does what you want.