The constructor for ARS is package-private (no modifier), which means that custom RS implementations cannot call it. That is, calling super(...) fails unless you create your replication strategy inside org.apache.cassandra.locator. This obviously isn't ideal.
I propose adding protected to the constructor in ARS. The attached patch was against trunk however I believe it should work for 2.1, 2.2, 3.0, and 3.x.