I've got a very bizzare 1960's semihollow that had a similar problem. Several of the tuners would tighten up part way through the turn. They were the kind that had a cover on the back, so I opened them up to find that someone had pumped a load of grease into them to try to solve the problem. The issue turned out to be with the worm gear its self. It was very slightly off center, so as it turned it would rub on the tuner cover. I added a paper gasket between the tuner and the cover (made from heavyweight resume paper, I believe) and cut it down on the outside so you couldn't see it. It seemed to add the necessary space to let the worm gear turn and free up the movement of the tuner.
Of course, I have no idea if this will apply to Mustang tuners, but it worked for me in this particular instance.