which takes the place of the hairnet...

But who does she hand the crystal off to or how does it get into the cup (or pie if you're in that camp)?

The way they show it on screen is that Sansa glides her hand over the top of the cup when she picks it up and hands it to Tyrion. But if things are consistent, then we know she didn't do it. QoT was sitting just off to Margaery's left... which is where Marg put the cup after she took it from Joff when the pie arrived. That seems the most likely time to me.