@Tempolo: From what I understand, the strings you add are hex encoded from a settings file. If you look in EFI studio, when you add an EFI string, the settings are at the top, and at the bottom you can see the encoded hex values. In the case of ethernet, what happens is that a property is created, which writes ethernetinbuilt as true, then this property is hex encoded for the bootloader, and loaded at startup. In a way it force sets ethernetbuiltin as true.
And i do not believe it is possible to get timemachine to work without the ethernetbuiltin set as true. (btw, this value is generated every startup, at \Library\Preferences\SystemConfiguration\NetworkInterfaces.plist)