I'm trying to figure out a way to update NetBSD on one machine
  and install the new kernel and userland on other machines.

See pkgsrc/sysutils/etcmanage, and read the scripts that are in
there (BUILD-NetBSD, INSTALL-NetBSD).  It's a bit rough, but you may
find that it does a fair bit of what you need.   I build on one
machine and install on another all the time with this.   I build full
releases, all the way to CD, but I have not found any good reasons to
want to avoid doing that.