What you want is the dog whistle shirt, that means something comprehensible and positive to most, but with a meaning that only the "right" people will recognize.

The Pioneer Plaque above seems like a great one.

For the recent Atlas Shrugged movies, I put little gold dollar stickers on my lapel. Not ideal for the general signal, but a good dog whistle. The movie "events" were quite laughable. For the last I thought I was going to get an entire large theatre to myself. I took a picture of the empty theatre before the movie started. Maybe a half dozen people finally showed up for the movie.