Correct, it is only enabled for GET requests. The only way to turn it on would be to edit the RestKit code. Generally it would not be expected to purge orphans in response to a POST because POST should be used for creating items and the response would contain only that item.
You would need to do some subclassing and handling of various operations to add code into the end of the mapping process before the context is saved.
You could consider observing NSManagedObjectContextWillSaveNotification
, but you should be aware that RestKit will be processing changes on a background thread and that is were the notification will be processed (this should help you, but you should be aware of it).