I managed to store custom data in Rhino objects UserDictionary of the active document using a Set method of Rhino.Collections.ArchivableDictionary. I can also retrieve the Rhino.Collections.ArchivableDictionary.Values. Data is definitely stored in objects UserDictionary during runtime.
Problem is UserDictionary is empty after saved Rhino file is reopened. Is UserDictionary meant to be saved with the file like UserText? Are there an additional steps to this process or is there an alternative simple approach to the whole issue of saving custom data with document objects in Rhino files?