According to this document it should be working, but looks like the inline definition gets written to the attributes only after using it by name in the 2D script, and is not accessible from 3D this way. We will check if this is a bug or some technical limitation.
You can put the definiton in a MASTER_GDL_….gdl file (plain text file). This way the attribute is created when the library is loaded, and you can use it in all libparts by name.
Library Platform, Software Engineer GRAPHISOFT SE