So then you must keep 'libexec' on the root partition (if your system is partitioned). Whereas '/etc' is on the root partition for sure.
And that's why I also mentioned en passing that sourcing local files from a potential /usr/pkg/libexec/rc.d/ directory would be problematic.
In fact, even today, making rc.d files automatically work from installed packages is a problem precisely because of this: those files currently end up in /usr/pkg/etc/rc.d/, and /etc/rc cannot use them by default because /usr/pkg/ might be in a different partition. There have been endless and repeated discussions about this issue on the NetBSD mailing lists.