the editor needs the transactional editing domain already
to make changes to the pictograms model (the diagram).
So even if you do not have a domain model which is
(EMF-) file based but just in-memory you would need to
provide a transactional editing domain for
manipulation of the diagram model.

For opening an editor programmatically you may have a look at
the CreateDiagramWizard class.

If you want to visualize a non-EMF model make sure you deliver an instance
of IIndependenceSolver
with your FeatureProvider. There, you have to implement
a lookup facility for your domain model objects.