Why bother drawing attention to them? As you say, there are usually a couple of old guys standing there passing time. If you stood next to them with a placard, 2 things might happen:If they bothered to even look, passersby would conclude you were with the two men (non-JWs don't care enough to make distinctions) and/or you'd confirm to the JWs what apostates are like.
Don't go drawing attention to something that no one - except former JWs - even notices.