For stateful services in a Service Fabric application, do read operations on reliable collections ever use the secondary replicas?
That happens in the communication stack. When using remoting, you simply just have to specify the kind of replica you want. Example:
ServiceProxy.Create(serviceUri, patitionKey, TargetReplicaSelector.RandomInstance);
Keep in mind, this will take bandwidth away from replication traffic.
https://azure.microsoft.com/en-us/documentation/articles/service-fabric-reliable-services-communication-remoting/