Yeah that last link to change the login screen works. It opens system settings yes - but as GDM user which is what the login screen is run under.
So What you are doing is changing the system settings for the GDM user - so once the settings opens click background and choose a background and it will change the background of your login screen.
It doesn't however change the login box which is probably what you are more interested in doing, as am I.
If you look at this thread http://fedoraforum.org/forum/showthread.php?t=264942
The post by rupertpupkin shows that this is possible - however he has been uncommunicative since posting that - perhaps try hassling him or PM him as I have done unsuccessfully.