Sorry misler (I was a bit harsh before I edited it), but we are splitting hairs here.
OK, the 330 is operated to 3 hr etops normally. I'm not sure if they were within the 3 hr range to La Reunion or not (they may well have been), but I suspect it would be academic anyway. Perhaps they had permission from the regulator, or it wasn't technically a "charter" - I don't know, and I don't imagine anyone actually cares.
Flight planning would indeed be interesting. As someone who has done real-time SAR I imagine it would have been "fill 'er up" and then work out a min diversion fuel to somewhere, wherever that may be. You then just fly the search pattern to your min diversion fuel (if required) plus a bit of a fudge factor, and hope that you find the survivor before you need to divert to some crap-hole in the middle of nowhere.