You could use neth.host.mynet I’d think. Not sure if there averse effects to such a hostname. It wont work properly if you want to enable the mailserver on internet for instance.
I am not sure how strict the two dots are enforced. Chances are only the gui will check that. Maybe setting your hostname directly in /etc/hostname would work. Worth a try I’d say.
Edit: This would need to be a custom template tho, as I just confirmed the hostname is set through the e-smith system. Not sure which template to edit how. Maybe the more knowledgeable Nethserver people can chime in here.