What about the 'hand over the cup' move? I still don't see how you saw the necklace thing in real time. I watched that woman's hands and I didn't see any movement like she pulled something off her necklace or that she had anything in her hands. When she left Sansa to go eat some of the food she paid for, her hands looked completely empty.
It's split second right before Joffrey goes to cut the cake. You don't get a clear shot of it happening, but it's the only time that makes sense to me. Margaery is the last person to touch the cup before Tyrion hands it to Joffrey the final time, and she clearly puts her hands over top of the mouth of it.