I think anything that includes a way of killing existing FTP instances is dangerous…
lftp is not only used for imaging, it’s used for transferring uploaded images from /images/dev to /images, it is what reports how much disk space is being used on the server, the size of images, deleting images (which takes a long time on Ext3), downloading the kernel and init…
I could just imagine some poor tech coming in to work early to update FOG and… right at the end of the installer when the new kernel and init is being downloaded, a cron event fires off and destroys the FTP transfer… and the poor technician has no idea it even happened…
You’d have to schedule this way way outside of operating hours; having it run AT the start time of the day is dangerous, having it run AT the exact end of the day is dangerous.
You might try time-based bandwidth shaping to slow down the fot-to-fog transfers over your WAN, you might get a much more stable and reliable method of controlling it that way.