Depending on how big the lump is, it might dissolve with soaking the chicken's butt (or chickens' butts) in warm water. Massage it under water and see if it breaks up like a clot of mud would. If it's really caked in there, you might be better off snipping the affected feathers, poop and all. Shorter feathers would be less likely to collect the poop too. It really helps to have another person to assist with holding the bird.
Realize that since you are dealing with, um, poop, you will want to wear gloves and clean and sanitize the sink or wherever you soak the bird. (You also have to get over the "ick" factor, if this kind of thing grosses you out.) Go slow, be gentle. Read some threads about giving a bird a bath for more info on that part of it, and how to catch them, calm them down. Good luck! Let us know how it goes.
Realize that since you are dealing with, um, poop, you will want to wear gloves and clean and sanitize the sink or wherever you soak the bird. (You also have to get over the "ick" factor, if this kind of thing grosses you out.) Go slow, be gentle. Read some threads about giving a bird a bath for more info on that part of it, and how to catch them, calm them down. Good luck! Let us know how it goes.