That's a great idea.
I may be wrong but I think I remember seeing "axle protectors" long ago. They were kinda like those foam pool sticks/floaters or handlebar pads except they were on the axle and sold as "saving your axle from bumps and scrapes." I'm probably going the shrink tubing route.
The axles widest point is 1.25 inches.......so inch and a half ones should work. With so many different options, oldskools idea of powder coating in a zinc type finish is a great idea.