They make enclosures and split-duct that will cover over an existing drop and d-marc for telephone, so these will definately work with a CATV drop with plenty to spare. They are already tampered and have a keyed lock installed. I really wouldn't install a shock or inertia based device, then again, I'm doing this as a pro and a false for something like that is far more likely and really isn't a high gain in security IMHO.
We've got overhead and underground here, and plenty of locations with no basements as well.
The easier way honestly, as I've done it on installs where a redundant communication line isn't being installed, is to relocate the service to inside the house, as if it's overhead, there's a point where it should be easy to pop it into an attic or somewhere else. If it's underground, then it's usually only a couple of fittings in worst case to move.
The thing is, that while this is a deterrent, don't be lulled into the false security, because, like the car alarm story, all they truly need to do is knock out a pedestal on the street or even on your property and it's the same as cutting the lines. The best solution is to have a backup communicator that works if the lines are cut.