> /usr/sbin/named (i.e., the bind8 installation that's part of the NetBSD
 > distribution) loads just fine at boot. My guess is that /usr/pkg/sbin/named
 > is failing at boot because it tries to run before /usr/pkg actually gets
 > mounted (though I see no errors during boot and nothing in any log files
 > says "/usr/pkg/sbin/named not found" or some such).

your assesment is correct.  I believe the startup scripts will silently
fail if named isn't there.
 
 > What's the best way to fix this? Should I force /usr/pkg to mount earlier
 > in the boot process? If so, how do I do that?

yes, you need it to mount earlier.  Putting

	critical_filesystems_local="/usr/pkg"

in /etc/rc.conf and that should do the trick.  See rc.conf(5) for more
details.

-Todd