Hi! Have you ever seen the Animal Planet show called "It's Me or the Dog"? It's great! Victoria Stillwell is the dog trainer and she's very good. Everything is done with positive reinforcement. There was a show that had a situation similar to yours. It took 2 people to do it... but it was basically teaching the aggressive dog that when they didn't attack that it was a "good" think and that they would get rewarded with a piece of chicken. Here's her website - it says you can watch episodes on there - if not maybe email her for some info.
Good Luck!!
Official Victoria Stilwell Website - author & host of 'It's Me or the Dog'