Snakes have very powerful intercostal muscles (the muscles between ribs, pretty much all the muscles snakes have, lol!!) and can force things up from fairly low in the respiratory system if they are still strong enough. Still, lots of junk accumulates in the lung bottoms where they have little or no chance of clearing it.
Still, only the constrictors are strong enough to force up much mucous and that drains them pretty quickly, they can't usually maintain that strength for very long.
The Zombie Mama is here!