I really don't think that's the case. Not anymore.
There was a rather large audience, and it would have been almost impossible for no one to notice Olenna putting a bead into Joffrey's chalice. It'd be a very risky, inelegant way to assassinate Joffrey, and it doesn't make much sense for Olenna herself to commit the act. It's also poor timing for House Tyrell. Margaery was very close to becoming a fully realized Queen, and I'm pretty sure there would have been plenty of opportunities later on to murder her husband.
The Fool gave Sansa the necklace to gain her trust - a dual purpose is possible, but unnecessary. Consider that Littlefinger was conveniently waiting nearby, prepared to receive Sansa, when it wasn't even guaranteed she'd wear the necklace to the wedding. You would think Joffrey's assassination was guaranteed by some other means, no?
Besides all that, think of how these things were/are being written into place: there is a scene showing Olenna has an eye for necklaces, Littlefinger reveals he specifically had the necklace made a few weeks ago, she takes a bead, Littlefinger brushes the necklace into a boat, and all the while Tyrion is preparing to undergo a trial.
I could be completely off-base here, but I'd be surprised if that bead didn't have some purpose down the line. It's potential evidence. It's possible the maker is traceable, or that the bead's material originates from a particular island (The Eyrie, perhaps?). Perhaps the full necklace will be found, and that bead will confirm it's connected to Sansa.
Now, did Olenna have a role in Joffrey's assassination? Very possible. I think the role of the bead is probably being misinterpreted, though.