I was thinking that sandpaper backed by neoprene (mousepad material) and backed up by a piece of hardwood would do it. I have used that method to convex knife edges by hand. I clamp the knife in and use it as if it was a file. It doesn't take that long to do a 22" machete by hand.

How about a red scotchbrite wheel on a drill to give it the finial finish? That would be sweet.