Andy--I never got anything out of eccentrics, really. The torque really wears down your elbow if you aren't quite strong enough to control your rotation yet. Of course once I actually got to the point that I was doing one-armed partials, I did a slow full range negative with each rep.
I did OAC work 2-3 times a week, with some bodyweight pullups on the off days--usually a set or two of 10 slow and controlled reps. I did like one-finger assisted OACs, and included them sometimes after weighted chins in the last month or so of my specific training. Generally I'd just do a rep with each finger--4 reps total--alternating arms back and forth without resting. Everything was highly autoregged, following a specific plan isn't a good idea because you'll have to watch your elbows pretty carefully. I think that working instinctively is another reason I didn't get tendonitis or even any elbow pain while training.
I believe that using a pulley can be very helpful once you are quite close. There's no point using a pulley if you have to put more than 20 pounds on it. The simplest method here is putting a rope through a weight plate and hanging it over a bar. I tried these for a while last summer while working towards a one arm (pronated) pullup and got very close before I had to quit while travelling... and this fall I focused more on other things so I still haven't gotten the OAPullup yet. But doubles and triples worked well for the pulley assists, just like the weighted chins... singles worked better for the partial and finger-assisted reps, in my experience.
