@Lukaz You could try doing a post download script that takes the hostname you set for that fog host and put it in the /etc/hostname file.
I think there may be other locations that hostnamectl sets on newer linux builds, but this could still work.
I haven’t done a linux post download script but you can see some other examples in these posts, mostly windows based, but I’m sure we can apply the idea to linux without too much trouble. Just gotta mount the client disk in FOS (Fog operating system that you boot into over the network for the imaging process) after imaging is done and inject the fog hots hostname variable (not sure what the var name is off the top of my head) into /etc/hostname on the client. It’s a bit of work initially, but once setup should just work from then on.
https://forums.fogproject.org/topic/7740/the-magical-mystical-fog-post-download-script