I don't know if you can do this, BUT...

You *can* put up easy instructions on the console, like "type 'halt'
at the login: prompt to shut down" and set up a halt account with
/sbin/halt as its shell...

"Mike Pelley" writes:
> I'm putting together a NetBSD machine to run some custom software in a
> non-unix literate office, designed to run without any intervention.  The
> only problem is that if they want to move the machine or something they
> might have to power it off, and of course they will not be logging in to
> type 'halt'.
> 
> Is it possible to watch for someone pushing the power button on an ATX
> machine with a smart power supply, and properly halt the machine and shut it
> down?  I didn't think so, but I don't know enough about apm stuff to be
> sure...
> 
> Thanks!   Mike.
>