Are you on i386 machines?  If so, then the easiest way for you to 
get a working serial console is to recompile the boot blocks with serial
console support set to the default.  The kernel will then take its console
from the arguments the boot blocks send to it when it boots the kernel, and
you should be able to control everything, and see everything, through the
serial console.  The attached pc keyboard is unusable in single user mode
and, unless you explicitly enable multiple vtys on the pc keyboard and
screen, won't be of much use in multuser mode either.  I've done things
this way for years and all of my NetBSD boxes have serial consoles,
exclusively, except for one.  And, yes, I was able to use ddb from those
consoles.
-Brian

On Aug 24,  8:22pm, "Tomas T. Peiser, CET" wrote:
} Subject: Re: Need quick way to crash NetBSD box.
} On Thu, 24 Aug 2000, Brian Buhrow wrote:
} 
} > 	Why not just modify /etc/fstab such that the entry for
} > the root filesystem is corrupted slightly.  this will cause it to panic on
} > mountroot, which should be a good test.  Then, boot off a floppy, mount the
} > root, modify /etc/fstab to be correct, and you're off and running again.
} > 
} Thanks, but I still must have something wired wrong. All off the messages
} come up on the Serial console and I'm still able start X and work away
} with attached keyboard. When mangled the fstab and rebooted I couldn't
} access the machine from the s-console when the ".... or sh to
} continue" prompt came up.
} 
} Any thoughts?
} 
} ----------------------------------------------------
} Tomas T. Peiser, CET  / No, I am not cynical,
}                      /  I am a sysadmin.
} http://bofh.nb.ca   / - conversation with D. Maxwell
} -----------------------------------------------------
} 
>-- End of excerpt from "Tomas T. Peiser, CET"