Communicator does not have a flat/faceted shading renderer as part of the Communicator::RenderingMode enum.
However, it is possible to get flat shading by removing the normals of the source geometry via the Converter option --sc_discard_normals true. Please be mindful of the limitation that by removing the normals of the geometry, it will not be possible to toggle on Phong shading in the Web Viewer.