Some kind of stapeliad or rhytidocaulon?
A forum buddy in far-off Oz asked me to help ID what might be a stapeliad, since a search didn't turn up any exactly similar pictures on Google. It's the long droopy plant, top shelf, in the middle, with the white flower bud about about halfway down.
Another possibility is Rhytidocaulon tortum...?
Thanks from two continents!