While in Basecamp classic you were able to mark todo-lists or messages as private (only visible by your company), it seems that there's no such feature in the "all-new" version of Basecamp.
I know that you can set permissions for whole projects in way that only your company is able to access the project and that you can move whole todo-lists to another project. But that's an annoying and complicated workflow.
Is there another easy workflow which simulates the Private Items feature from Basecamp classic?