Speed limits for towing smaller trailers mostly derive from safety concerns about overloaded or imbalanced trailers being unstable at high speeds. A battery-only trailer with little or no cargo space, designed and certified in conjunction with specific tow vehicles, could easily be safe enough to operate at highway speeds.

I think the main reason why we don't see anyone seriously pursuing the battery trailer idea is that it would be an expensive niche product. It would have to be mostly a rental-only product, and offer few advantages over simply renting a more suitable vehicle.