Backing up a trailer, is a skill that is not intuitive, and gets more difficult as the length of the arm between the hitch and trailer wheels gets shorter. This one aspect of the e-wheel concept would seem to be a show stopper.
Initially I was thinking that lifting the E-wheel or E- trailer off the ground could solve this problem, but when you consider the down force required for good traction, the only thing that makes sense is to have the batteries generate the down force with their weight. Lifting 800 lbs of batteries would bottom out the rear suspension, and is not a safe or reasonable option.
If the arm between the trailer hitch and the wheel/wheels were able to hinge horizontally in the middle under servo control,while supporting the weight vertically, the rear wheel/wheels would be steerable independent of the vehicle wheels.
With todays electronics, this should be easily accomplished.
A front wheel drive system from a car with the CV jointed drive shafts and steering would offer another way to steer the E-trailer wheels.
The control system would want to automatically keep the e-wheel/e-trailer centered behind the vehicle while backing up. A rear view TV camera would allow a better view of the e-wheel/e-trailer.

The concept would apply to any trailer, and would only be required when backing up.
Now to build a prototype.
This link of the autobackup generator trailer for the T zero is exactly what I have in mind:
autobackup trailer video