Can Grease Pencil work with 3D Scene/Objects in Viewport for Grease Pencil? Is it possible to use a modelled a hall, corridor-- throne room etc as background whilst using grease pencil for viewport working? So allowing you to have the 3D environment as background whilst you use Grease Pencil for the 2D animation?
Yes. Grease pencil objects exist in 3d space so they can be used in scenes together with traditionally modelled 3d objects.

This video shows regular 3d models being used as a reference in a 2d animated scene: https://vimeo.com/164067136