That certainly is one way to keep broodies, and I'm glad it's working well for you & your birds. It's a method worth a try for anyone else's flock, but it's not always guaranteed success. I think it would work best if you're available to check on the hens in the nest boxes many times a day. Otherwise the broody can leave the incubating eggs -- either to go take a break or because another hen bumped her off the nest -- and return to a different nest box & begin setting on those eggs. Meanwhile, the eggs that were beginning to be incubated may be left to go cold if there isn't anyone to sit on them.
Laying hens like to follow trends, and a nest with an egg already in it is very attractive to them. "Look at that" they say "other hens think that's a good place to leave an egg, it must be right!" A nest with a brooding hen is especially appealing. "Wow, she thinks that's a great place to raise her chicks, it really really must be a great nest site! Move over, lady!"