I've always wanted to know how difficult is it to sing while being up side down? Remember Peter Gabriel? In one of his concerts he stands on a magnet platform in special shoes up side down and sings. And as far as i remember the guitarst was also up side down playing. I also watched one cool video where a pianist was playing up side down (can't remember the name). How hard is it? Is it very different from normal playing and (or) singing?
It really isn't a whole lot harder than playing or singing right side up. It is just stage performance. It is in the best interest of performers to interesting things. The normal challenges associated with being upside down would continue to apply.
Playing and singing upside down would certainly present some challenge but that would most likely be due to the discomfort of doing something odd. All of our bodily functions like swallowing and pooping work fine upside down so singing and playing guitar upside down for a few minutes should be ok.
That being said, I just tried it and it was not that hard. If you are physically able to you can probably try it on a pull up bar or off of a piece of furniture.