I love this idea! It has to be something distinctive enough that an NT wouldn't mimic it unconsciously (a real worry) and enough so that it would be recognized. It has to be something that won't alert NTs, though, if it is done.
There should be a separate but similar salute for allies, such as parents.
Here's my suggestion.
Aspies: Left forearm vertical, right index and middle fingers stroke quickly wrist to elbow (approximately) twice. (Note that by Aspie, I'm referring to anyone at all on the spectrum.)
Allies: Right forearm vertical, mutatis mutandis.
The benefits of having such a salute are numerous. We can communicate without the NTs around us eavesdropping (well, they could eavesdrop, but they wouldn't hear us say "I have Asperger's"), and we know whether we should ask for tips on preventing sensory overload or suggest that they read Pretending to be Normal. (Presumably, we already know they have a greater than average chance of being an Aspie. The question is whether they know it or not.) We can communicate it faster than speech, and possibly more easily. It demonstrates that we're not faking, as if that were a real worry, but still-- you know this person has done research and is on WP, whereas someone who claims to be an Aspie may just have heard about it on TV.
_________________
I'm using a non-verbal right now. I wish you could see it. --dyingofpoetry
NOT A DOCTOR