We're considering working on a new feature that will allow "mounting" part of the namespace of one ZK cluster into another ZK cluster. The goal is essentially to be able to partition a ZK namespace while preserving current ZK semantics as much as possible.
More details are here: http://wiki.apache.org/hadoop/ZooKeeper/MountRemoteZookeeper
It would be great to get your feedback and especially please let us know if you think your application can benefit from this feature.
Alex Shraer and Eddie Bortnikov