I saw rawhide hammer that was very well used and was almost ragged
and worn like leather. Apparently what you should sand the off the
faces and soak it in water to expand the ends and then as it gets
used the faces soften up and are gentler on the metal.
I always put a new rawhide mallet in a vise, and then use a coarse
file or rasp on the surface to remove the glue surface. I've never
wetted the leather--interesting idea.....
You might be interested in paper mallets, which Charles Lewton-Brain
has helped re-introduce to the contemporary market. I think Allcraft