Using shapes as region comes out of the box, because they are derived from ContentControl which has a default prism adapter. However, if you want to use RadDiagram as a region a custom adapter should be implemented. I'm attaching a simple solution demonstrating how to use shapes and diagram as regions.
Hope this helps. Please let us know if you need more info or face some difficulties.
All the best,
Hristo
the Telerik team
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