Ah ha...thats a more imaginable situation...I've no idea ) I guess if you were willing to soldier something to the photo button, you might have a chance, but then again there may be a difference in delay times from the camera opening to the flash firing...it would be interesting though if you figured it out. I don't think you'd need a fancy trigger like that Pocket wizard plus 3, just the most simple radio or light type like nikon...
If you were after the same effect as that photo above I think you could recreate it with multiple exposures, I think that shot was shot in normal bright daylight and a gradient was placed over the entire image and then area where the couple are was just brushed out of the gradient, and their shadows were brushed in stronger. You could do something similar with multiple exposures and just brush out the area of interest.