Hi there!
I had a 13 week old pullet with nearly an identical injury today. The break was in the same location, just under the nostrils, and nearly broke her upper mandible off. Really, it is almost exact!
Anyhow we rushed her to an avian certified veterinarian about 2 hours away because Kua (the pullet) was in very obvious pain. You could tell; she'd open and close her beak and then shudder and close her eyes. She is usually very rowdy so I knew she was in pain. There are a lot of nerve endings that deep in the beak.
I wanted to let you know what my vet said and did in case it can help you with your Nugget!
The vet stabilized the wound by patching it with dental acrylic. This is an (obviously) surgical safe material, very hard. She used it to attach a small fiberglass piece over the beak. The idea behind this is to stabilize the break so that it could not move, become worse, and also to help seal it against infection. It is a deep type of wound right into the core of the beak, which is bundled with living tissue and nerves, and is very prone to infection when the break is so deep and bad.
Right now Kua looks like someone stuck a tiny piece of bubblegum to her beak! I will take photos tomorrow.
(I won't say how much it cost. Lets just say that I might have the most expensive chicken in my state right now.
)
My vet said that two things will happen, either:
1. The beak will heal over the period of 2-3 weeks and it will regenerate the damaged areas and all will be well (though it may not exactly grow back because of how high up the break is, so any misalignment may always persist) or
2. If the nerves and blood supply have been badly damaged, the broken part of the top of the beak will be taken by necrosis (the living tissue will die) and it will fall off and not regrow.
IF I can just get her to eat. She cries in pain each time she tries, even with the softest of foods like babyfood. I feel so terrible for her. I have a prescription for antibiotics and painkillers that I administer twice a day. I will keep you updated....