I won't try to answer the stock/broth question...
Take your carved out chicken/turkey carcass and dump it in enough boiling water to cover it, reduce to simmer and cook 20-30 minutes...
Remove the carcass, and in this case you'd want to strip whatever fragments of meat off...discard the bones and cartilage, which are pretty much "done" for food value at this point...
You might want to run the stock through a sieve or collander to remove any unseen bone bits..you can always return any meat scraps afterwards, seeing this is for your cat...
I'd try feeding this to the cat in very small amounts until I was sure its "constitution" was up to the possibly "runny" effects of this twist in diet; if all else fails, you can always "tweak this up a bit: with parsley, garlic, etc, freeze it and use it yourself...
Lifter