How can I customize the background of the FileVault login screen in 10.8? I've successfully changed the background image for OS X login screen, but cannot do it for the FileVault login screen.  This makes sense as the contents of the disk are still encrypted at that point in the boot process.

Per this link: superuser.com it appears possible, but I honestly cannot understand what they are suggesting here.  Any ideas or interpretations?
There is a workaround to get your custom background image copied onto the FileVault login screen without having to mount the encrypted partition and copy it manually, the workaround is detailed in this guide.