After speaking about this to NetDocs support, although secretaries can created ndmail folder mappings for their Lawyers (delegates) - this is actually unsupported!
It does work - secretary has' full access' to laywers mailbox, goes through their subfolders, mapping them to ND. If the secretary or lawyer drags emails into those folders, it files for both.
However, as it's unsupported, it means that asND continue to develop ndmail, this option is not being included/tested and may not work in future versions.
I think the ability to map folders for delegates (and allow mailbox owner and delegates to file emails into them) is really important - particular in Law Firms where there is a secretary/PA - Lawyer relationship