OP,
A lot of people on this site will never carry their gun all day long through, and up-and-over whatever. They will not have scraped it against a rock or whatever else.
With that said I don't see a practical need for painting an AR as its finish is quite good. The finish is starting to wear through on my hunting AR after about 20 years of actual use.
An old, blued rifle or shotgun with a very worn finish I think is a good candidate for trying your idea. If I hadn't already painted my hunting shotguns I might try it myself. However, I'm not sure a liner-finish is as corrosion (rusting) resistant as paint, but I sure as hell like the idea of the good grip it would afford and if it has insulating value that would be a huge plus too.