If you take a rake or something and re-arrange the bedding without adding scratch, will they do just as much scratching? It might be worth trying.The way I use scratch is actually to encourage the hens to move more, and simulate foraging behavior. I don't use much - just one handful - but I scatter it all over the run, so they have to go and look for it. My run has a lot of logs and things, and a shrub, so the chickens have to go under, over and around things. They will turn the bedding upside down inch by inch looking for every lost grain of scratch, and will do this all afternoon, just in case they missed someI'm not sure if the calories from the scratch cancel out the exercise of foraging for it, but that's the most reliable way to get them to move.
They might spread it back out, not find enough goodies, and stop looking. Or they might find enough little bugs or whatever and keep scratching anyway.