I keep thinking about this, but I can't help thinking it would just look ugly. I've seen a couple with spare wheels on the back and they've just looked wrong.
All the aftermarket rear bumpers I've seen so far all look a bit too industrial for me too.
I would love to get my boot space back though so I would be very interested to see what you come up with.
I keep wondering about just storing it vertically inside the book like the Classics, but never got round to doing anything about it. That at least would give a slightly better layout.
Guy