I often use the feature to import a DWG into an Inventorpart. While using the share view option. These 2D-Layer is not exported, so only the 3D-Parts.
A workaround is to use Navisworks. Combine 3D Inventor Model and 2D-DWG in Navisworks, export to NWD and upload to Autodeskviewer will show the DWG. But it would be a nice feature to have 2d-DWG exported into Autodesk Viewer directly with Inventor.