Saw it in one of the "gripe" threads.

A working solution (I use it myself): pull out the rear flip shelf, and put the belt buckle with the loop on the left pivot pin, then push the shelf back into its place. Voila - your belt is now between the "fixed" side shelf, and the flip-up one - no more flaps. The downside is that it _slightly_ blocks the interior trunk light.

 

I have stowed mine inside the rear bodywork.

(I don't know the official names of all these parts, so descriptions will have to suffice.)

First, remove the plastic cap from the place on the C-pillar where the belt goes through its little loop. Then remove that bolt. Now, you can pop off the plastic piece near the rear window where the seatbelt retracts into. The belt and tab can be rolled up and stashed inside there. Put the plastic piece back on, and the belt won't be in the way.

The downsides: Now there's a hole in my C-pillar where the belt bolt used to be. I haven't covered it up with anything yet.