Argh Jackson has this problem too and I'm at a loss what to do. He's never bled, but he chases and chews his tail. I've put deep heat on it and he doesn't even notice it. He too goes into a trance when doing it.
I didn't think the amputated tails anymore. If I thought it would stop Jackson doing it I would probably try and get it done.
My trainer did tell me not to yell at them when doing it though, just distract them with clapping or a loud noise of sorts. Then when they're calm they get the reward.
It hasn't worked for us so far but Jackson lived in a house with a constant drug abuser, so I have no doubt his brain is a little fried. It's frustrating.