They will probably choose a calm secluded bay in the Mediterranean, with high statistic probability of low winds and as difficult to fly from the US as possible. Maybe somewhere in Greece? Anyone knows calm windless bays in the Mediterranean?
The south of France, Corsica, Sardignia, Baleares and northern Italy and Spain on the med side are way too risky for wind conditions.
Greece is mostly too windy as well, unless you are in a lee of a big island where the problem is gusts.
South Italy and south of Spain are possible. Valencia, Naples or Palermo for example. Malta is probably too exposed, but would fit your criteria as being difficult to get to from the US.
Venice or other cities in the north of the adriatic sea are good candidates: Flat water, usual low wind, close to Switzerland... As close as a lake as you can be. If it's good enough for X40, why not AC multihulls...
Some place in northern Africa may be suitable, there are some nice quiet areas in Tunisia for example, but they may lack the necessary infrastructure to accommodate two 100ft multihulls monsters...
