One of my hens got dry fowl pox last month. I though it would go thru whole flock, too, but it just got the picky eater and a few spots on the bottom hen. I give my girls onions and spinach in addition to regular food. When I started making Penny, the picky eater, eat some onions and added Braggs's ACV to one of the waterers, it stopped progressing and then got better. It had been spreading to her face and I didn't want it to get in her eyes or turn wet. There are some scary pox pictures when you start looking.
I don't know that the onions made that much of a diff, but the hens that regularly scarf them up didn't get it. Mine don't like garlic, which is another thing to add to help.
I also gave them her eggs even tho it is safe to eat for humans, I figured they could all use the extra nutrition.
NutriDrench in one of the waterers would also help support their immune systems while they fight the pox.