Meanwhile, this has been implemented. According to https://www.jetbrains.com/help/rider/Exploring_Type_Dependency_Graph.html you can

• Select files in the project view and get a diagram for all types they contain
• Get a diagram for search results (like "find usages") which allows you to get a diagram from basically everywhere

Use Ctrl+Shift+Alt+A and select the diagram or click the diagram button in the "Find" toolbox after you did something like "find usages".

For OP's usecase, one would probably have it easiest by selecting all files, then press Ctrl+Shift+Alt+A or select "Diagrams" from the context menu.