I'd advise against this.

Remember, the bearing balls themselves are accurate to a few thousandths of an inch. A crown race of any quality is going to be ground to the same standard. It also is meant to be an interference-fit on a concentric round seat. I doubt the ability of anyone to hand-file such a seat. No doubt the race can be forced onto whatever polygonal shape results, but just as the seat isn't round - and like as not not concentric either - the race won't be either, forced out of shape and location by the irregularities of the seat.