from my limited treatment of horses, I was under the impression that tea tree oil for this in horses can be quite dangerous? don't know why it would be dangerous as it may be something I would try in humans but was definately told it can be dangerous for horses?????
there are several things you could do externally for the infection part but again, I am not sure it would work on a horse where you can't cover the lesion as the treatment if not covered would probably attract more flies.
try this bloke www.holisticvet.co.uk/ he is quite a few miles away and I am not sure he has a practice nearer to you but maybe worth an e mail to see if he can help. I have only treated horses for tendon/leg/muscle probs before so not much use apart from the web link I am afraid!