I realized that the only chance of saving her was to stitch the wound closed. Unfortunately, so much was missing that I couldn't stretch the skin that far. So, I did the best I could.
The first couple of days were difficult for her, and I wasn't sure she was going to pull through. Large areas of the surrounding skin turned an awful blue-green color. But she recovered and is doing well.
Five days later, she is happy and active, and her skin is now a healthy pink. There is much healing yet to come, and for now she must be segregated from the other chickens, who readily attacked wounded members of the flock.
A month later, feathers continue to fill in nicely, but the hole in her head remains.
Update: It is now over a year since the attack and surgery. She remains healthy, and it is now impossible to see any scar tissue anywhere. Remarkably, she appears to have either grown feathers in the scar tissue, or somehow shrunk the scarred area.