Mike, I took the photo to be a message moreso than a demo of usage. As in, "hi can you people please look into 'yin and yang'?"
Yeah, I more or less agree, Jonathan. It didn't mean much and the various signals of "yin-yang" are/were pretty widespread in Asian martial-arts (as you already know). Ueshiba would have known that, too, so I'd take it more as a signal that he was in the club, too, rather than a "here's a valuable clue" sort of thing. Same thing with a lot of the references in his douka..... using the correct words was an accepted way of titillating the readers who didn't know, while at the same time being a signal to the cognoscenti that his style adhered to the classical Yin-Yang, internal-strength, etc., dicta.