Awesome, whether it's a scenario for a role-playing game, or a story you're writing, I think it's a fun premise to explore!
If zombies are so useful, then the feudal lords would have hoarded them and not let the peasants in on the action, but some peasants probably would manage to keep a few zombies in their herd, or maybe if the zombies are dangerous, the peasants are forced by the feudal lord to be their handlers...
Yes, maybe it will become both rpg and a story. I have too many plans for my own good, but as a way of exploring the universe a pen and paper RPG is a good way. The premise I have come up with is so bleak that I will have to try it out to see if it will be attractive at all.