Interesting idea. I don't think there's anyway to make it significantly unobtrusive.
"the middle of the belt arranged up under the rear fender somehow"
That's the problematic part. If it could be folded without damaging it you might pull it off. But the belts are of pretty stout construction. One of the biggest PITAs when installing a new belt is getting it to curve tight enough to wrap it around the small front sprocket.
When I travel I carry all the tools I'd need to swap out a new belt, except a jack and a spare belt. When my belt broke in '05, I was blocks from the dealership (~120 miles from home). With a borrowed jack from the dealership, and a loaner bike so I could run to the Harley dealership, I was able to replace the belt in a few hours.
Not ideal but doable. If I'm not near a shop? Well, that's what road hazard insurance and my cell phone are for. I know I can get a replacement belt from Harley. I don't think they'd loan me a jack, though, so I could install it in their parking lot.
