Pax, can I please have some more info?! I might be able to help you with some info! Where is the wound? Is she an adult or a chick: Has she been laying recently? Is she eating and drinking? Is she listless?
Hmmm. usually eggbound hens are listless... Is it possible the poop just got caught in her butt feathers? We have a BR with an extra poofy butt that gets poopy really easily. Though that pic looks nasty... could you post clearer ones perhaps?
Cant really tell from the picture what's going on. Separate her and bring her in the house so you can see better and observe her. Could be a partial prolapse, in which case, a hemmorroid ointment may help shrink tissues. Could it be a wound? I just cant tell.