Comment 2 for bug 144798

Revised external hooks proposal:

Add 1 extra boot parameter (which replaces find_preseed, override_script, and override_files)

custom_installation=/path/to/custom/installation/folder

In casper-premount, a script looks for the custom_installation folder, copies it to /custom_installation, copies the preseed file to / for initrd preseeding and runs casper_premount.sh. In casper-bottom, a script runs casper_bottom.sh and then moves /custom_installation to /root/custom_installation. Further hooks are used within ubiquity. The structure of the custom_installation folder is as follows.

/custom_installation
/custom_installation/preseed.cfg #the preseed file is loaded via initrd preseeding
/custom_installation/hooks #the following scripts are executed as follows:
/custom_installation/hooks/casper_premount.sh #executed as soon as the casper folder is copied
/custom_installation/hooks/casper_bottom.sh #executed within casper-bottom
/custom_installation/hooks/early_command.sh #executed at the beginning of the Ubiquity installation
/custom_installation/hooks/late_command.sh #executed at a later stage of the Ubiquity installation
/custom_installation/hooks/success_command.sh #executed at the end of the Ubiquity installation

External files and packages can be placed anywhere within the casper folder, it's up to the hook scripts to use them.