It should be possible as root to look at / update user units for selected users. Especially since some of these users might not have tha ability to log in at all (system-like users).

The use case that led to this bug report involves enabling and starting a user unit for a 'pulseaudio' user running a 'pulseaudio.service' unit in order to have a network sound server. The 'pulseaudio' user is set to have no password (and this cannot login) and  starts at boot because it was configured using 'loginctl enable-linger pulseaudio'

What I wanted to do was:

    systemctl --user=pulseaudio enable pulseaudio.service
    systemctl --user=pulseaudio start pulseaudio.service
    systemctl --user=pulseaudio status pulseaudio.service

(but the --user option do not take an argument)

What I tried to do instead was:

    sudo -u pulseaudio systemctl --user enable pulseaudio.service
    sudo -u pulseaudio systemctl --user start pulseaudio.service
    sudo -u pulseaudio systemctl --user status pulseaudio.service

(or some other variation using 'su' or 'su -l' instead of 'sudo')

That didn't work because the XDG_RUNTOME_DIR was not set. This could have been caused by either (or both):

* /etc/pam.d/{su,su-l,sudo} not using the pam_systemd.so module at all

* pam_systemd.so not setting the environment variable. I suggest you look at
  bug reports like https://bugzilla.redhat.com/show_bug.cgi?id=753882 or
  bug #70810

What I ended up doing was something like:

    su pulseaucio -c '
      XDG_RUNTIME_DIR=/run/user/$(id -u)
      systemctl --user enable pulseaudio.service
      systemctl --user start pulseaudio.service
      systemctl --user status pulseaudio.service
    '

I don't loke it, especially setting the XDG_RUNTIME_DIR that was. This is implementation specific and not portable. That's why I suggest you provide a way to select the user session on the command line.