When fetching and locking external tasks via the REST API it is already possible to only fetch local variables for an external task.

However, it is not possible to configure this when creating an External Task Client via Java. (https://github.com/camunda/camunda-external-task-client-java/blob/master/client/src/main/java/org/camunda/bpm/client/impl/ExternalTaskClientBuilderImpl.java#L68-L73)

There should be an easy way to configure the external task client to only fetch local variables when locking external tasks.