I'll tell you, I had one chick that has a hard dried clump of poo under her vent area too.  Like yours, she could still use the bathroom, but a little more poo clung to the mass each time she went.  I could NOT get it off with just a warm rag.  I actually ran a few inches of warm water in the sink and sat her in that (holding her soothingly in it, because she was afraid).  But once she'd been in it a minute or two, she actually seemed to enjoy it.  I had to sit there with her soaking and work soak over the clump.  I got MOST of it off.  Then I had DH assist me in clipping the rest off (I was too nervous to try it alone...was afraid I would cut her because it was so close to her skin).  After that area was trimmed up, I never had another issue with it.  Good luck!