Thats a possibility, huge power requirments, but then again it could only engage when the tow vehicle loses traction.
I think it's very do-able. With traction sensors on the tow vehicle the trailer would only provide drive when needed. Power could be supplied by a battery that had a direct 4 gauge connection to the alternator of the tow vehicle.
I don't think you need motors as large as they used on the Mini, so the power requirements could be scaled down for the trailer.
Having computer controlled traction sensor would be a huge improvement over the PTO or hydraulic systems as 6 wheel drive would only be used as needed. You may even be able to use the same approach Subaru uses where power is shifted between the axles depending on the traction at each axle.
The slick thing about the electric hub motor is you can provide different amounts of power to each hub, so traction control could be individualized to each wheel
Just a dream right now.