If you're using fuel as ballast then ZFW is still ZFW

(a) AN, when they converted the Goose from pax to freight upped the MZFW by changing the fuel usage procedures to keep outboard fuel for wing bending relief .. effectively using fuel as ballast ... ah, memories of a wonderful time ... Launy Mt Barrow departures ... sightseeing trips HBA-LST of a Sunday morning after delivering the papers ... etc .. etc ...

(b) problem with routine use of fuel as ballast is if the fuel arm is strongly variable and you end up burning the fuel .. so long as you know the story for your aircraft, all is well .. or, at least, you know when you are getting into deep do-do.