I have had 3 horses with tails like you describe over the years. Bought them all years after the injury when it was all settled. Two of the three didn't even have any white hairs. It didn't seem to bother them at all except if I tried to pull a tight tail bandage over the bent / lumpy bit, so I either unwound the tail bandage or used a velcro tail guard. I always wondered what happened to make them like that.
Sorry I realise this post isn't strictly relevant but would be interested in other people's comments.
I also had a problem with my dog with a baddie tail. The vet said that there are three main blood vessels going to the tail and if there is a problem they can die and the tail would have to be amputated...... Luckily it went all ok and he has still got his tail, bless.
My old TB ex-racer had this - we think his tail was trodden on while he was rolling. Basically the vet cut all the hair off below the bone to reduce the weight, bandaged it to straighten and support it - and a few weeks later it was good as new. Took ages for the hair to regrow, though.