A friend rather better than I am at following the news just alerted me to this article, which presents a new theory on Massys' Grotesque Old Woman. The article suggests that Massys' modelled his Duchess on a real person: an old woman with a severe form of Paget's disease.
The drawing attributed to Francesco Melzi (above) could be a copy of a Leonardo drawing, which is now thought not to be an original. The theory is now that Leonardo, or possibly Melzi, copied Massys, not the other way round. This does seem possible, since the drawing is not as detailed.
Though this may initially seem to solve the mystery, it actually adds to the mystery by raising a couple of questions. First of all, who is Leonardo's drawing (above) of? It appears to have been done quickly, so could have been an observational sketch. Unfortunately the top of it is chopped off, so we can't tell if Leonardo's old woman had the distinct headdress or if she was the same as Massys'.
The painting is displayed next to the old man painting (shown in an etching above) for the first time in a while, in the exhibition 'Renaissance Faces' at the National Gallery.