I agree that a difficulty ranking for dogs would not be worth the time to compile it due to the huge differences in dogs abilities and personalities. I love taking my dog with me but I would not take her on anything higher than class 2+, and I also worry about some of the rougher talus routes. I also agree that you should be prepared to carry your dog all the way down if it comes to that, so that affects some of the routes I can take her on.
Having said that, I think a list of all 14ers with dog only specific notes would be helpful. Stuff like "sharp talus for 3 miles" or "water available year round at x and y on the route". Or even something like the trailhead reports - "80lb lab made it easily", "Corgi had bleeding paws at 2.5 miles" etc. Even without that, all the photos and trip reports that are here have made it pretty easy for me to figure out what my dog can do. The photos are especially helpful. There are a couple routes I want to do and see for myself before I take my dog, where it would help, but the trip reports make most of the routes a clear cut yes or no.