That's awesome!
We have one that we've been working with for about 12 days, which is how old it is. It had a leg like that, except it stuck out more to the side, not the back. Same thing basically though.
We started with the VetWrap and B-complex as usual, but after two days, it was like we'd done nothing. Put her in a narrow cup for half a day. Nothing. Tried the vet wrap again for two more days, and minimal improvement, but we got a better idea with a rubber band. After two days of that, we found it seemed to help more because she was more mobile with it. We let her run around for a day with nothing after all that, and she was much better, but still slipping that leg out on occasion. At least she could walk. We've got them on horse bedding pellets so they aren't causing slippage, she is. A couple of days ago, we put a new rubber band on her looser or wider so the leg can't kick out, but she'll exercise it more. She's running around fine like this, but this time I want to leave it on for about 4-5 days, hoping that leg won't kick back out.
I'll get a picture tomorrow as they're all sleeping now.
You'll have to let me know about this cast. I'd be interested for this chick if this doesn't work.