Using a crate is not mean IMO. When broody, a hen eats and drinks significantly less than normal. Over time, this leads to a loss in condition and can result in the bird being more susceptible to illness. Extended broodiness is not a good thing. When a bird decides to stop being broody can vary. I have read here on BYC that some birds can literally starve themselves to death as a result of being broody (this may be an exception, but you get the idea).
If possible the crate should be inside the coop. If not, then its a case of ensuring that your run is predator proof and that the crate is protected from rain / sun. I'd imagine that the higher the crate is elevated off the ground, the safer your birds will be.