Hold lines in figured bass are as far as I can discover not so well documentated.
A hold line in the lowest figure is easy but in middele or upper figures not so easy or unpredictable (for me).
In those cases I have the impression that writin the full harmony and afterwards replacing it with the hold lines is succesful. Look at my example.
One will need it not so often but in Bach’s St Matthew I know some places…
Though I don’t intend to make a new edition;-)