Had to find out from pjones that livecd-iso-to-disk already had EFI support.
Created attachment 368042 [details] Document --efi
This package has changed ownership in the Fedora Package Database. Reassigning to the new owner of this component.
While this exact patch wasn't used (or things were change a bit afterwards), --efi is now documented. There may or may not be a backport to F12.