Here is the hub you mentioned. I didn't even know this was commercially produced. I don't know how dangerous it is (to your motor or your knees if on the pedal side), to shift while moving at speed, but that seems to be the product you want!
So, if the Sturmey model can be driven backwards, and you have the motor chain turning it, how do you rig the pedal side? Are you using the rubber rag joint cog on the right side? A 50 or 54 tooth road chain ring with the 36t rear rag-joint cog should do just fine, if you use a front freewheel, and a rear derailleur for a tensioner. Still pretty scary because a derailed chain might still pedal you! Or chainsaw your leg! And tandem bike parts are expensive.
That is why I said if you can machine the hub to take a standard BMX freewheel, you could have it made, for purpose of this thread. I think it would be quite a high ratio though, on the motor side, in all 3 gears.