How do I reference a diagram in a DSL T4 template?

Question

Google's not coming to my rescue, here, and I just know this is the perfect place to ask.

I'm writing a custom DirectiveProcessor for a DSL and I want to be able to access a diagram from within my T4 template.

Within my DirectiveProcessor, I've loaded the domain model and my diagram using (wait for it...) LoadModelAndDiagram(...).

So, now the diagram's loaded into the default partition in the Store, but I can't for the life of me work out how to resolve a reference to it later.

Can anyone guide the way?