[Serious] Could an omnipotent and omniscient god prove that he was God?

[Belacqua]

Is not the real McCoy, if He exists, capable of pulling off the same hat trick?

I guess that depends very much on what you think the real God is like. Obviously Spinoza's God wouldn't undertake the kind of kenosis you describe. I think it would be fair to say that Spinoza's God "couldn't" become a person. Omnipotence doesn't mean that God could go against his own nature. 

The threads that people start on this forum nearly always address the anthropomorphic emotional God from a literal reading of the Old Testament -- the concept of God that Blake called Nobodaddy. So the argument is that God would think and act like a big powerful person. I agree with you that if God were like that, he would likely perform demonstrations as he does in the Old Testament.