Zero-Fill format before installation

[Bullet 446]

I have a remote machine with KVM access that I'd like to format and
reinstall FreeBSD 7.0. I've succeded in formatting /dev/ad6 but not the main
drive, /dev/ad4.
Is it possbile to zero-fill format during the installation instead of simply
deleting and recreating partitions?

I've tried booting into single user mode without any success:
# umount -a
umount: /cdrom: not a file system root directory
umount: /var: not a file system root directory
umount: /usr: not a file system root directory
umount: /tmp: not a file system root directory
# dd if=/dev/zero of=/dev/ad4
dd: /dev/ad4: Operation not permitted