Hi Marcie,
How about the Vulcan sign or a loud Klingon "Kaplah" . . . oh wait that would be nerdy folks . . . just kidding.
To be honest, I prefer to let sleeping dogs lie. If a trans person recognizes me as one of the tribe and decides to come up say howdy (secret handshake aside), I would respond in kind but that is me. There are many, some who may not be comfortable in public, don't want to draw attention prefer not to be recognized or just want to blend, that even a simple secret sign could be unnerving.
Hugs
Isha
