Any chance of a link or pic of said tensioner? Do you mean the types that single speed riders use that are static and don't move? I don't see how you'd mount that with a rear derailleur, but I'm sure there are ways.
(Not my bike or image; http://farm4.static.flickr.com/3101/...5498cb.jpg?v=0 )
I use lizard skins chain stay protectors... Mine is yet to wear or show any signs of giving up soon.