I've used my extension cord on many flights, from AA to ANA and have never had an issue. I don't know of any FAR that prohibits extension cords from being used. I have the extension 3 prong connector that attaches to my macbook plug and it is a long annoying wire and I've never got grief for using it. I just think it's a misinformed FA who made up their own rules and contempt of FA has become a violation.