Bactroban is what they gave me when my skin fell off my forearm from a severe liquid burn. It would probably work well for her head. Also, Aquaphor, the clear stuff that they sell for babies butts and skin which is really a healing ointment/skin protectant, would help keep the wound moist so it doesnt dry out. My wound was much larger than your chickens and it healed fine with these two things. I dont know if they have it or anything like it were you are, though.
We had a chicken who was torn into by a predator, she had a large hole in her back and you could see everything inside. We dont have any livestock vets around, so we packed it and changed the dressings and used healing ointment and stuff on it and she survived. It was pretty disgusting though. But you don't necessarily need a vet in order for her to survive.
And depending on where her scull is showing, she may not have had muscle there in the first place. I know that humans do not have muscle on the top of their heads, but try as I might, I can not find a diagram of chicken muscles anywhere. They cant move their combs around, so they may not have muscles around there. If it looked like she did at first, it may be because the other muscles were inflamed from the trauma and covering the area, and have now gone back to their normal size. Just something to look into.