I think this is an interesting idea and would like to hear how it works out. Just wanted to throw in this tidbit: we sometimes use bead-blasting to straighten thin parts before doing other operations. In principle, it's the same idea as shot peening, where the compressive stresses tend to curve the part concave on the side that gets peened. If you do use a *light* blast and your master is thick enough, it probably won't change the flatness significantly, but if you notice that after blasting, your master is high on the ends and hollow in the center, this would be my first guess about the cause.