I've heard various speculation about the stone key in other threads but since this one exists I thought I'd ressurect it.
I was rereading TKP and had a strange idea. It says several times that
She [the Red Queen] will return to claim her throne bearing the lost staff of the Red Queens and that will be the sign for us to rise against the slavemasters
While not exactly a staff, I'm wondering if this refers to the stone kay Elspeth was given by the Sadorian overguardian. Although it is assumed that it was lost when Dragon's mother died, it is possible that it eas actually taken with Kasanda when she left the first Red Queen.
Since we have, as yet, seen no sign of the real staff (although it would be fitting for Matthew to find it) it would fulfill both prophesis at the same time.
On a slightly different track, if we assume that it ws lost with Dragon's mother, it could also be somehow used to control the Entina. It was only after the Gadfian slavers gained control that the Entina was 'awakened'.