The FAIme service now supports uploading a custom shell script. This script is added to the installation ISO and gets executed during the first boot on request. You can upload a plain shell script or a compressed version using gzip. Using the postinst script you can adjust the new installed system to your local needs after FAI has done the initial installation.
The FAIme service is a web service, that creates customized installation images by just a few clicks. It's also possible to create custom cloud images via the web interface.