I remember reading somewhere that umpires for slam finals are often (though not always)from the country of the slam. I think it was Enric Molina who said it when asked how he felt umpiring for USO finals.
I always wonder this also.
I heard during Rogers' match vs Berrer in Basel that the chair ump for any given match cannot be from either of the players countries. So for this match, they couldn't be Swiss or German. But I am not sure if this goes for TMS & GS. or DC.
just my 2 centavos