Well here's my take on stuff like the human flag. Take this BBoy Junior for example, go youtube him if you don't know who he is. He mastered walking with his hands because he lost one of his legs. He could do planche push-ups (feet off the floor) and other feats of strength and balance. That's because he has been doing it forever. So, if you really wanna pull off a human flag, just keep going at it. Same as how most people do their first pull-ups - you keep trying until you get it one day.
All that aside, you need an insanely strong core that can support not just your own weight, but more. And you have to let your arms get used to handling your bodyweight in multiple angles for different lengths of time.