Maybe something like this can prevent the worst case scenario:

In INSTALL pre:

[ -e /var/service/agetty-tty6 ] && sv stop /var/service/agetty-tty6
ln -sf /etc/sv/agetty-tty6 /var/service/rescue-tty6

In INSTALL post:

rm -f /var/service/rescue-tty6
[ -e /var/service/agetty-tty6 ] && sv up /var/service/agetty-tty6

That way, even if all standard TTYs are removed during install, there is at least the rescue TTY.