You are right that there is no straightforward way to achieve the required behavior. However, there is a possible way to hide the header of the container shapes.
You can try extracting the default template of the RadDiagramContainerShape control. You can follow the steps described in this article.
For your convenience I extracted it along with all resources that it uses in the attached file. Once imported into your application you can try setting the Visibility
property of the ContentPresenter
" to Collapsed
Please give this approach a try and let me know if you need any further assistance.
Pavel R. Pavlov