Sturmey does offer a hub that would be a clever mid drive . it has a sprocket attached to the spoke flange.
then there are 2 more gear ranges..
But the rear dropout is still a short vertical , have to add something for the torque washer to grab onto.. maybe there is a reaction arm to get that function.. then the dropout slot does not have to have that secondary function.
Ig Hubs rotate backwards in there in some ratio combinations and the drive torque has to be backed up, somewhere..