I've found some good-looking clips from JagWire for cable routing but does anyone make a clamp-on cable housing stop?
Do a google image search with string "problem solvers backstop". It seems to have been designed originally for top tubes but I don't see why it wouldn't work with a seat tube. I haven't used it myself though so I don't know how well it would work.
Edit: Just noticed that they are for brakes and not shifters.