Hello Peter,
1. If there are no info related to block, but the diagram itself, I just recommend you build a custom file type and save your additional data together with diagram. For example, save diagram data, and right after save your custom data. I don't see a reason for adding such specific functionality in diagram. Alternatively you can just inherit from TatDiagram and add additional published properties in that new class.
2. Unfortunately there are no such events.