Fezbox uses the Kickstart feature that has been available in RedHat for a while now. Sadly it doesn't support FTP yet, see the Kickstart readme file, it should be in the 'docs' directory I think. So if you want FTP support, bug RedHat with it, or write it yourself :)

Cool thing about FezBox is that it creates the ks.cfg file and then includes it in the image file. Neat.