Would this just be a few lines of code? In ParallelWriterIndexCommitter:114 plus having a method on ClientKeyValue that copies the KeyValues and returns the new List<Mutation>?

      final List<Mutation> mutations = (List<Mutation>) entry.getValue();
      // Pass the list through to the KeyValueBuilder to potentially make copies of them, as
      // the memstore requires the KeyValue to have a backing buffer
     mutations = kvBuilder.copyMutationsIfNecessary(mutations);