The only thing I've seen so far that approximates Armor as DR is Sleeping Impergnium's Grim N Gritty (GnG) Rules. It turns armor into a statistcs called Protection, and uses a concept of Defense like in d20 Star Wars. The system also allows for called shots.
The system is a little clunky, and really isn't geared for low fantasy; it states all this in a disclaimer, though. If you're looking for something that gives a harsher feel, GnG can do that. It might also provide a good start from which to make your own set of Armor as DR rules.
Unfortunately, Sleeping Impergnium is no longer at RPG Host (they used to be a freely hosted site), so I don't know where you can find the file. I have it at home (not here), else I'd email it to you.