Good question, as I'm currently faced with this same dilemma.

FWIW, you can easily tap the non-threaded hole to 10x1 to accept a Campy RD, without appreciably widening the bore. This way it remains backward-compatible with Simplex RDs. However, I don't know whether the Campy RD will pivot properly since the Simplex dropout lacks the Campy pivot stop. I'll be following this thread to see what other advice people have...