I had this idea about using the "ReadinessProbe" of the pod to define which one should be used by the service. Like

            - wget
            - -qO
            - /dev/null
          failureThreshold: 3
          initialDelaySeconds: 10
          periodSeconds: 10
          successThreshold: 1
          timeoutSeconds: 10

So in a mirror the backup member will not be ready and the main member will be ready and receiving service connections. 

The problem here is that I would have to manually change the service's selector and chose a better Rolling Update strategy for the STS (since new updated/recreated pods would never became "ready" without the main member dying first).

Well, that was just an idea...