The only way I've been able to think of is a Kong chew toy full of peanut butter. Usually barking all day is because of separation anxiety.(or so I'm told.) a puzzle chew toy with a treat keeps the dog's brain of it's separation anxiety.

Dogs are social animals that hate being alone. So they get really worked up.

The only other humane solutions involve the dog not being alone. (I.e. having a companion animal or the owner's return.)

Unfortunately that first solution would involve breaking into the owner's house. Throwing a random chew toy in there and hoping nobody notices.

The other cause of incessant barking is some "intruder". This could be another animal or person inside or outside the house. Nothing much can be done in that situation either.

Once a dog thinks he's the alpha of the "pack"(yes that includes humans). And he feels its his roll to defend the "pack's" territory, there's nothing short of a zombie eating that dog's brains thats gonna stop it barking at "intruders".(this is why behavior training at the puppy stage is so critical, it prevents this sort of problem.)

I have a limited understanding of doggie behavior. Always been more of a cat person for that very reason.

Sorry not helpfull.