Sorry, but the InMemory repository does not support Retention & Holds. The predefined types from the spec for retention and hold are not exposed from the InMemory server by default. If you create such a type afterwards, the server will recognize the type and properties but it will not know the semantics associated with properties like hold. In fact you shouldn't try to add predefined types (those starting with cmis: ) at runtime because their presence indicates if the functionality is supported or not.
For testing retention and holds the InMemory server is not an appropriate server. The InMemory server is limited to basic functionality.
Hope this helps...