I had to forge my association head's signature for my first IBJJF card, because I was in Brazil and we didn't have internet, or a scanner, and he was in a different city in Brazil. I can't remember if I get his permission first or not. I don't think I've ever had to get another signature when my card lapses to reregister, but I'm not a black belt.
But I know other people that forged signatures out of convenience for out of town instructors, and it caused a shit storm because the IBJJF actually followed up on it and called the instructor haha.