Cannot access Diagram metadata via GTL or VB Script

I'm using PD 16.5 SP03 PL03.

I need to extract a CSV file that contains selected metadata from the entity attributes that are displayed in a LDM diagram. Nothing works.

If I create a Generated File containing GTL, no preview tab appears in the diagram properties (this approach works for the model and entities, but not for the diagram).

If I write a simple VB Script to extract metadata via the DisplayedObjects collection, I get an error message that says the Class doesn't support automation.

Is there another way?