Hi George,
That's true, diagrams don't inherits from Extensible, as you can see in MetaModel Help. Therefore, you cannot extent them.
The GUI is poorly offering you all these capacities although they are useless for Diagrams.
That said, some extensions have been added however, and you can work with extended attributes and VBS (not GTL).
For instance, you can create a read-only computed extended attribute of type Text, called XPreview.(avoid name that can conflict with metamodel names).
You can add it in a new Form (named Preview)
This computed attribute would contain call to your collections
The "%Get%" output value will be displayed in the Preview form.
But you must take care of metamodel limitations concerning diagrams. Also a diagram does not hold object, only symbols.
So to access object data, you must use the Object property of ObjectSymbol class.
Good luck
Marc