Make and ally - keep the gig
Hey TB,
I've run into this quite a bit. Many times, when you're brought in because of a unique skill set that is not available in-house, you're viewed as a "gun slinger" and a threat - often people feel they'd have been capable of delivering on your assignment.
But, I always approach these engagements as a knowledge-transfer opportunity. Rational people can "get" this, recognize the op, and take advantage of it. Those that don't are generally obstructionist, a trait that's usually apparent to their direct supervisor.
I try to get these people officially assigned to the project. This way, they have deliverables for which they are responsible. If your help is required to get them done, they tend to recognize your value to their "survival".
In any event, I always report, in writing, on a weekly basis, whether required or not. There is always a way to include instances where someone is being an impediment to progress (without naming names) that will eventually get the attention of their manager.