What's a nifty way to restrict my dog run without re-installing it?
They make clamps that you can very easily install onto the line. They come in a range of sizes to fit most any line size. Here is a picture of what they look like.



You can procure these at most home supply centers and at any decent hardware store. Look for Steel Cable clamps. 

There is another type of connector that may be an alternate choice. These, as pictured below, will be found in the electrical supply section of the same types of stores mentioned above. They are typically made of copper and so will not rust. Price will be a little higher though.