A bud tried one promising idea that didn't quite pan out. He had a bunch of the orange stick-on targets (don't recall the brand). Sprayed them with Pledge or some other spray wax and let them dry. Then he hit them with black spray paint. The idea was that the wax would keep the paint from adhering tight, so it would flake away around the hole and make it easier to see, just like the S-n-S.
His problem was that the paint flaked off too easily as he was peeling the targets off the backing. Really unpredictable, and with smaller stickers another problem was that the ball impact blew all the black off the orange background.
He was headed back to the drawing board last time we talked about it, but since I haven't heard more, I'm betting he hasn't found a solution.
Dunno what else he tried, but I'm betting there's a solution out there.