I vaguely remember those. I think the nuts are held by a sheet metal retainer clip. One of mine spun, too. From my foggy memory, both can be replaced if you drill/Dremel them out. You could also epoxy a nut into place or put in a nut-sert...
Is it part 20 on this diagram? That's the nut held in place by the special retainer...
http://www.realoem.com/bmw/enUS/show...diagId=51_3899