Off the wall idea: is there any reason why a conforming item can't have extra bones/groups? Think of the thigh becoming the equivalent of a two-fingered hand, one finger being the rest of the leg and the other being the holder for the kneecap. There's techniques like ERC which would let the normal knee-joint movement control this extra bend. So the conformer mechanism drives the normal knee joint, while the extra bone moves at a slower rate. And that group, the rigid plate, would be set not to bend. All I can say is that it sounds plausible to me. But all I've rigged has been nice, simple, mechanical devices.