Thanks for all the advice. As it turns out, the logistics of using a roof box for European Delivery don't work well. The folks at European Delivery have told me that they won't ship the car back with the luggage rack attached. I'm looking at the cost of shipping the rails and box separately from Antwerp, but I suspect that it will be too expensive to justify the extra storage space. I suppose my wife will just need to pack lightly this time.
It's a shame that BMW has discontinued the screw-down hardpoints. I took a look at the hidden doors on top of my 2000 328, and that seems to me to be a much more secure approach than clamping onto the rain gutter.
Once I get the car back stateside, I'll take my time to explore the bike rack options.