As long as you have them trained to use the boxes and not lay out in the yard, I would think it would work. I have never tried external nest boxes. But chickens can learn to lay anywhere you want them. The only down side to this might be that the eggs may attract predators. I would keep fake eggs in the boxes at all times and collect the eggs often. Give this external box idea a try and if it doesn't work, you can put them inside the coop.
I think you will find that your hens will lay wherever they have to when they feel the urge to lay. If you confine them away from the boxes you will get eggs layed on the coop floor and maybe while they are on the roosts. Better that they have free access to nest boxes.
Hello and Welcome To BYC! If you have them outside during all daylight/lighted hours it should work since they don't come off the roosts at night to lay anyhow. But if you are going to have them locked away from the nests at all when it is light, I would have at least some boxes inside so birds that aren't on the common schedule have a place to lay.