If you're concerned about flushing writes consider `mount -oremount,ro` -- this guarantees writes are flushed and is the only truly important step of a system shutdown anyway WRT filesystem integrity. Once filesystems are mounted read-only you can safely power off the machine.

Filesystems can be remounted read-only via the serial console as well, with a break-u. Useful even when userland is otherwise inaccessible such as in the event of a fork bomb.