The plywood moulding method was invented and used by German aircraft manufacturers as early as 1915 indeed. I doubt that they used balsawood as sandwich filler. There is a subtle but important difference to the Mossie construction technique however.
The Germans simply patched thin veneer onto a mold which bends nicely onto the curved contours. When it comes to plywood which itself is made of numerous veneer layers things become complicated. Plywood doesn't bend as nicely as veneer does. Bending it onto a compound surface is a challenge. Steam treatement helps to a certain degree but has its limits. Therefore the surface needs to be cut into numerous patches which can take the bending. This results in gigantic puzzle. To make things even more complicated the joints of every puzzle piece must be scarved at an exact angle to offer sufficient glueing surface. An enormous amount of work.
I pull my hat for the Mossie team.
Here's another project which faces similar challenges
www.projekt-arbalete.ch