The issue with coatings and lubricants is that you’d need to use a nontrivial amount for them to be effective, and that means the patient is at risk of aspirating the substance. That happens, and boom, you’ve got aspiration pneumonia. Even with some kind of encapsulated or solid gelatin coating you’d have to be very careful to make sure it doesn’t leave any kind of residue or break into pieces that could be aspirated.