The meat would probably be really tough. I'd think the only acceptable preparation would be braising or confit. They probably have a lot more parasites (worms) than you'd be used to as well.
I stayed in Kauai in 2012 and the locals said they do not eat the chickens. They said the meat is very tough and doesn't taste very good. They also said the chickens are useful because they eat millipedes, which are everywhere. They claimed that the sting from being bitten by one of those millipedes is unbelievably painful, and feels like a gun shot. I'm not sure about the gun shot part, but everyone seemed pretty serious about how painful it feels.
There's an ant called the bullet ant, so named because its sting feels like getting shot. Its Wikipedia page shows other similar nicknames in Spanish and in Portuguese. So, given that other bugs' stings have already been described as comparable to getting shot, I believe it when you say that they say the millipede's bite feels equally painful.
The localsn in Maui have a recipe for cooking the feral chickens there. Put the chicken in a pot. Add a lava rock. Fill with water. Boil until the rock is soft.
Those chickens have a lot of genetic legacy related with old practices of cockfighting. Being aggressive also helps with survival.
Races of chicken that were breed to fight are very slim and all muscle. Not much different than pigeons. Probably a hard meat. Old roosters of most other races are perfectly edible.
Of course locals could have more than one motive to not want foreigners eating the birds. Specially when there is an old and widespread tradition of let the chicken free-range around villages