That sounds an awful lot like an injury that I had on a goose - he had a 'tongue' of skin torn from under his 'chin' down to his breast. Just a literal tongue of it, the bottom 2 inches or so was missing. I cleaned under the flap and on the skin, then did the stitches along the sides but not at the bottom. If you leave a stitch missing at the top, you can just stick a syringe or baby's ear syringe in the top with water with a little iodine (til very light tea colored) and flush that out, let it drain out of the bottom (hold paper towels there in a gloved hand so you don't soak her) then you can irrigate the wound when needed. It might be that you may have to cut the skin flap off if it starts to die. If it's green and hot and doesn't smell bad, then it's still at least circulating. If it's green black, smells, and is cold, there's a problem.
Also if you had to do that all again, slather neosporin both on the skin under and the underneath of the flap.
Now that you've done this, don't use any more hydrogen peroxide because now the skin will want to try to heal to itself and thus you won't want to burn it anymore; today was a definite exception.
Note that when she heals, there will be adhesions (where the skin adheres to the muscle layer underneath) and it just can't be helped really. There might also be scarring. My goose, who as I mentioned was very very badly injured, healed but with a little keloid like scarring where the 'tongue' of skin ran out at the bottom (well - it was eaten). And because I had to pull it a bit to get it to fit, the line on the back of his neck is crooked. You might recognize some odd things like this with your hen if she heals as we hope she will - they won't hurt her health.
You can give 1/4th a cc if it's penicillin G procaine 300,000IU. The 1/2 cc won't hurt. It might be a nice start for her!
Keep an eye on the stitched skin - that's the part that is potentially problematic. By the way, were you able to cut the edges clean so that they heal better?
