Has anyone seen that if you create a new property in P3DLineGroup and then try to view that property in Navisworks, it doesn’t show up? Any ideas?
You can't right now. The list of properties shown in Navisworks is fixed.
There should be something posted on the app store soon that will allow you to get the data straight from the project, including custom properties.
There is a way to get your Properties from DWG into Naviswork, but this involves working with Excel spreadsheet which is far from ideal.
What we had to do was to produce DWFXs, and they bring all custom properties correctly. However this approach is not perfect, as in order to see your custom properties, you need to add your Properties to 'Piping and Equipment'. As a result you have to have same Properies fot Equipment and Pipe Run
Component. On the plus side, if you don't put anything into Property Value, it will not appear in your DWFx.