Trailers are each subject to regulations. Now, if you had one of these, you wouldn't need a plate because it doesn't have its own set of wheels.
Or a range extension pack? granted this would eat up storage space (read: longer trips imply luggage), and I'm not sure on the whole (battery) weight-to-range-extension ratio, but possibly one of those tow hitch cargo carriers topped with an aerodynamic 'box?' filled with batteries that plugs into the charging port to the car?
Just an idea? Obviously it would cut down on performance, and the added weight would weigh on range, but.... I guess a die-hard could always build one themselves (assuming you can even put a trailer hitch on a Mini of course). Weight would be an issue here as far as getting the damn thing up there, but you could also have something that mounts to the top, like a roof box filled with cells... granted, getting the damned thing up there would be a bitch... maybe something like this could be installed in the garage to lower it down???
I know its been six days since your comment, but I just came across this sub. I go camping with my jeep all the time and use this hitch with this cargo bag. This is the only pic I could find of my jeep with the hitch while attached (with the bag only half full), but I must say its one of the best purchases Ive made.