Based on my own prior research and this prior post
Thinkpad nvram in NetBSD I have been under the impression that a custom kernel was needed.
In
Arch Touchpad Wiki it is possible to disable the touchpad upon external mouse detection.
I also believe that some notebooks have a Fn Key to disable the touch pad.
You may be able to use something like
xdotool(1) run in .xinitrc/.xsession to disable when starting X.org. Note that xdotool is wip in NetBSD and the referenced manpage is from FreeBSD.