How do you return an entity property of 'type' => 'struct'
Based on entity_metadata_node_entity_property_info_alter() and entity_property_text_formatted_info(), I suppose the getter callback should be the most generic callback the entity API offers: entity_property_verbatim_get. 

This should allow you to access the properties as: $custom_entity-&gt;structure_property-&gt;sub_field1