Maybe pressure can? I've never had even the oldest bird (of mine, anyway) not get tender either in the crock pot or pressure canning, BUT, once upon a time...
When I was much younger, I was sharing a house in California with a friend, and we found whole chickens really, really cheap at a discount grocery. Being very broke, we jumped on this. They weren't very big, and a sign posted on the case said they were Rhode Island Reds. My friend said they must've walked all the way from Rhode Island, because they were so tough.
We boiled those birds all day long and half the night, they never did become anything edible. The texture was similar to truck tire. You could barely cut it with a sharp knife, much less chew the stuff. Rubber chickens they remained, no matter what.
You have my sympathy. My first trial cross, Brahma roo, Cornish hen, didn't turn out well. (I'll try C roo, B hen, later, just to see if that works better.) Pretty birds, but small. Only 10 of them, so not a huge feed loss. Some are pullets, too, so I'll just keep them for layers. The young roos are mostly not very big, only a couple even seem worth the trouble, but if I gotta kill it, I'm gonna eat it! Or if that's not possible, I have dogs and cats who'll like it.
Maybe you could saw the larger pieces of meat off the bones, and dry it for doggie chew toys. I bet they'd love it. Or run it through a grinder and cook it in a crock pot with brown rice for the dogs. I did that with venison trimmings, for mine, they scarfed it down.