WARNING: I have ZERO evidence whether this works. You may kill your YN560 in the process. I am sure this will violate whatever warranty YongNuo or the seller may have. Do this at your own risk!!!
The Nikon SB-800 normally has a swivel range of -180 degrees to +90 degrees. In other words it can only swivel 90 degrees to the right. There is a mod out there for brave SB-800 owners that will let you swivel the head up to 135 degrees to the right:
Being somewhat of a risk-taker, I did it to my SB-800. It was extremely hard to twist the head -- like a very stubborn jar. A couple of times, I nearly gave up (because of the mental stress of potentially breaking an expensive flash). But eventually the head turned. I did this mod a couple of years ago and have never experienced a problem. (Do so at your own risk!!! And of course be aware that the SB-700 and SB-900 can turn 180 degrees both directions without any mods.)
CAUTION! WARNING! DANGER! Do not try this at home!
Out of curiosity, I tried the swivel mod on my dead YN-560 (1st generation - with plastic foot). Let me emphasize it again: I was doing this to an already dead YN-560 (my YN-560 died when it fell in the water) so I had nothing to lose. If you want to try this on a working YN-560, do so at your own risk. Please note further that Nikon's reliability is way ahead of YongNuo. I'm pretty sure an SB-800 will handle more abuse than will a YN-560.
That said, I twisted my dead YN-560's head and found that it was pretty easy to twist it to the right. Easier than opening a bottle. I don't know what that means, and I have no way to tell if it damaged my YN-560 further. I also don't know if the 2nd generation YN-560 with metal foot is any different in this regard.
Anyway, now my dead YN-560's head twists 135 degrees to the right:
Too bad I have no way to know whether it could work...