One obvious drawback to adding a hood to a standard Bock or JoWo nib is the shape - the wings of the nib will force the section to be much wider than the sleek profile of the Parker 51.
JoWo have made "Arrow" style #5 nibs - I bought some in polished steel a few years ago, but now I can only see solid 18K gold listed at fpnibs.com where I bought the steel ones. They might be something that comes in and out of stock.
LINK to the 18K gold Arrow nib