Is there a way in FreeBSD to (being root) run a command as unprivileged user, like nobody? Kind of like reverse of sudo. Oh and considering that 'nobody' has /usr/sbin/nologin as shell - so su is not an option.
Answer
You can su to an account with the nologin shell if you use the -m option.
Example:
su -m cthulhu -c '/usr/bin/scorpion-stare'
will run the SCORPION STARE command-line utility as the user cthulhu.