I didn't yet try 'systemctl poweroff', but according the man page, 'telinit 0' is equivalent to it. Another one I didn't yet try (just discovered it in the man pages but I don't have access to the installation ATM) is systemctl's --host option, in other words, let systemctl do the ssh. Can this be expected to work ? If not, any other ideas ?
ssh host 'nohup systemctl poweroff &' -- дамјан