I know you can use 540 motors in electric helis. Hell, the IKarus Sport motor is not as hot as some car motors I have. I seriously doubt you'd get off the ground at 7.2V though.

My Eolo R22 is running the stock motor at present. When it died I'm going to try one of my Monster Maxx WIld motors from my TXT-1. They are rated to 14.4V. Should be intersting.