[CentOS] ligthdm shutdown without a mouse

I have set up a computer that's going to be hand-carried through several
airports on the way to its final destination.

This is the Lenovo laptop that I asked about earlier, where everything
works well except for the touchpad that isn't recognized at all.  (For
the touchpad I'm just kind of hoping that a future kernel update will
make it magically start working; for the time being the computer owner
will live without the touchpad and just use an external mouse.)

When carrying this though airport security the security guards will
likely require that this laptop be turned on for them.

How can it be turned off again properly after being checked without having to carry a mouse too?

Is there a keyboard shortcut for shutdown with lightdm?
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