use a subtle variant of these gestures to signal to other Effective Altruists who you are, without giving it away to more egoistic types.

Sorry, still not seeing it. Why are you trying to give these cryptic signals to other EAs at work? Is the idea that EAs will start cutting one another specially favourable deals and giving preference to one another for good jobs? That seems likely to be hugely counterproductive, because as soon as such things get suspected it's going to be bad for the whole EA movement.

If you're overtly signalling affiliation while making a political speech or something, why not just do it by talking about effective altruism? If it's a speech in which it doesn't make any sense to do so, then what the hell are you doing signalling affiliation in the first place? Again, this is the kind of thing that gives a movement a bad name. (Ditto, even more so, if it's covert.)

a subtle "Dark" gesture [...] a financial company

OK, so I am, let's say, an investor considering putting some money into a hedge fund. I go to visit their offices.The fund manager or one of his colleagues greets me by putting his hand behind his back and giving a thumbs-up gesture. Are you suggesting this is a subtle gesture that won't make anyone suspicious?

Again, maybe I'm just missing something. But every time I actually try to imagine a concrete situation in which this sort of gesture might be useful, I can't do it.