There is a great drug called CAPSTAR to use on maggots. Its a pill for dogs and cats to kill fleas.
Our vet treated our rooster for a bad maggot infested wound by grinding this drug up and irrigating the wound after all the bigger maggots were out.
It kills the larva too. Its available at feed stores, etc. I also read about giving it to wildlife to kill maggots.
Don't know about orally in chickens, but itseems it would be a very good last ditch thing to try in a very severe/advanced case in a favorite chicken.
From what I understand the maggots just begin to die and fall off in 30-60 min.
Good luck if you are reading this because you have a pet bitd wit maggots and hopr this tip helps.