If we try to add digital signature in dwg file we get this message that "You do not have the right to view, add, or delete digital signatures".It seems that Digital Signatures and Digital Rights are just not supported in DWG files.
More and more users are using digital signatures in DGN files and as this option is not available in DWG workmode their workflow using Bentley software is broken and they have to switch to AutoCAD.
Please vote for adding digital signatures for DWG. Digital signatures will be more used if it will be available and more universal, software should be ahead of progress not behind. Now similar as element templates signatures are not universal(missing/lost on convert) so are not widely used.