As long as you get the gearing right I don't see a reason it shouldn't work. I have a three wheeler that had the same concept from the factory. It was originally pedal powered but it had a Sturmey 3 speed hub mounted mid way between the pedal crank and the rear axle. Very much like your idea. The challenge, or the unknown here is if the Sturmey hub could take the abuse of the motor used like your idea would require. Good luck, let us know if you try this.
Tom