The first Oversized (and Left hand drive) hubs I made had 12mm axles. I didnt wanna cut the frame at first so I ground flats in the axles that were a tiny bit wider than they dropouts (just like Rob said). It was only really the axle threads I took off, and the peg still had the support of the larger cones and bearing behind it.
Id say cutting the dropout is always best, but if there is a reason you dont wanna do this, flats on the axle aint too bad, esp if youve got spare axles.