I fostered a dog who did this - my kitchen looked like a scene from a disaster movie as she was a huge dog. The dog had caused the original injury herself when in kennels (before she came to me) by biting her tail in anxiety. The self harming soon stopped but the injury took a while to heal. The vet warned it may be necessary to amputate the tip but gave me a series of plastic tail guards, which allowed air to circulate at the tip, but you taped on to the tail itself. It took a few weeks, but it worked. She would leap about, wagging her tail furiously when we came in banging it on radiators, walls etc but the tail guard protected it from further damage and it did heal well.
My greyhound broke her tail twice because of wagging (and if you got in the way it was like being hit with a whip) and it didn't even register with her. I only realised because her tail suddenly wasn't quite straight one day, and on investigating I found two kinks in it. We only had her for three years, but she wagged the same until the day I had her pts so I don't think the mad wag brigade ever grow out of it.
Our Whiffie (coined by scuttlebuttler - whippet/staffie) is insanely excited to see us when we come home, even if we've been out 5 minutes. We've taught him not to jump and he's working on it. We've taught him to find a toy for us to tug and he remembers that. BUT he whacks the end of his whip tail on the radiator and then streaks blood all over the walls and doors.
Now, the walls clean and the doors wipe down, but surely that's got to hurt??