After some research it seems that JCckContent class is still not campatible with object Free.
I'm not fully oriented in SEBLOD structures but to me it seems that new JCckContent[OBJECT_NAME] refers to the class stored in each storage location plugin (article, user, etc.). Unfortunatelly the class folder of free storage location plugin is still empty so JCckContentFree can not be instantiated.
Also, it seems that the create method of JCckContent class still stores more data to '#__cck_store_form_[content type]' and more2 data to '#__cck_store_item_[content type]' only. So no custom table :(
Again, I may be wrong, would be great if SEBLOD team comment