I believe that's already the default behavior of OS X. It will put the computer to sleep (ACPI S3), and start dumping contents to hard disk. If the battery dies

I believe that's already the default behavior of OS X. It will put the computer to sleep (ACPI S3), and start dumping contents to hard disk. If the battery dies or is removed between waking, it will read it in from disk. I'm not 100% positive on this, but I do recall reading a similar description.