I've not tried this yet Darrin, but I'm sure I'm much better at this than the concentric portion of the movement. For some reason, I really burn out quick at chinups or pullups (and it feels like it's my delts that get burned out first, leaving my upper back wanting more). The first set, I can do ten or eleven w/o weight (a couple more for chins than with pulls); the second set, only six if I'm lucky! What is weird though is that it feels like I can pull up alot of weight on the first two or three reps, but, after that, I start losing strength quickly.
But I've noticed I'm very strong on the way down, so I tried something the other day. While doing pullups at shoulder width; when I got up to chin level at form failure, I removed my left hand and slowly let my self down with just my weak arm.
I don't think I can meet or beat your record, because I don't have that kind of isometric muscle endurance, especially where my shoulders are involved, but I'll give it a try on my next workout that involves chins.