Some people litter train their tiny dogs so they will never go outside (like a cat) which I don't agree with. I met a girl who had a (seriously untrained) chihuahua who never put his feet in grass bc it "was dirty" and you can't have your precious accessories getting dirty.. Ugh..
BUT to litter train a dog to supplement going outside, I don't see a problem with. I would probably use a clicker or praise to train though. Treats might turn him fat or dependent (wherein he will refuse to go pee or poop outside unless he gets a treat which will make him hold it on his walks so he can pee in his box just to get a treat).
And dogs will not cover their litter like cats do either. Mama cat teaches the babies, and if the mama cat doesn't then you cannot teach a cat for its life. My sister's cat was not taught to scratch as a kitten by his mama and he's 5 yrs now. He only scratches it if my sister's bf is home. So dogs will not participate in that kind of behavior.
I'm not sure what kind of litter to use. Some people use cat litter to mask the odor and some use puppy pads. The dust in cat litter might upset a dogs moist nose, but I'm not sure.
It's worth a try ^.^ Couple that with his seperation anxiety training though
Keep us updated!