RAW does not address this directly, but the use of the word "banner," without supplying a definition particular to the game, implies the normal English meaning, which is a flying flag and not just something with your insignia on it. So clothing does not qualify unless you take it off and stick it on top of a pole. (Also I'm not sure a Small or Medium cavalier's tabard is itself Small, which is a RAW requirement. A Large cavalier's might be.)
