I think that the scaffolding is going to be erected earlier than April. Of course, I may be wrong. Suffice to say it's worth checking with a reliable source before making a trip to Himeji.

BTW - "Hideous wooden box" seems like an emotive way to describe the roofed structure that will be erected.... Are you suggesting that the restorers should use a somehow attractive box to cover the castle? And is it actually wooden? That would be an unusual choice of material. (Most of the temporary shells that I have seen around buildings in Japan are constructed with steel, plastic and composite materials).