Short of a WW2 style fight to the end campaign in 1919 pretty close to zero. Bureaucratic inertia is powerful.
BTW, what about keeping Prussia but also recreating Hanover from a part of Prussia? AFAIK, autonomist sentiments in Hanover were rather strong in Imperial German times. Not sure about Weimar German times.
In 1912, Hanover was the only German territory that belonged to core Germany that voted for autonomist parties:
And it was the same pattern in earlier German Reichstag elections. Here's 1890, for instance: