This should work ?
devfs -m $D/dev rule -s 4 applyset
devfs: open: $D/dev: No such file or directory

This does work :
        mount_devfs devfs $D/dev
        devfs -m $D/dev rule apply hide
        devfs -m $D/dev rule apply path null unhide
        devfs -m $D/dev rule apply path zero unhide
        devfs -m $D/dev rule apply path crypto unhide
        devfs -m $D/dev rule apply path random unhide
        devfs -m $D/dev rule apply path urandom unhide