This is the line I used for the late command:

d-i preseed/late_command string cp /cdrom/somefile /target/system/folder && cp /cdrom/someshellscript.sh /target/system/folder && chroot /target chmod a+x /target/system/folder/someshellscript.sh && chroot /target sh /system/folder/someshellscript.sh

I appended the line above to my preseed file. This line basically copies some script from the cdrom or iso to some folder in the target system. During installation the /target folder is the "target" systems root system (get it?).

So for me I copied all the files I need from the CD or iso to some folder like root or opt or even tmp in the target system. Then I changed root to /target and executed all the regular linux commands. Wrote a shell script that executes after all the copying is done to do what I want it to do.

Note:

Inside my shell script, I unset all of the installer's environment stuff:

unset DEBCONF_REDIR 

unset DEBCONF_FRONTEND

unset DEBIAN_HAS_FRONTEND

unset DEBIAN_FRONTEND

Then proceed accordingly.

Hope this helps.