Used a Z Drag to actually pull a raft or kayak out of a wrap or pin?
Yes, on numerous occasions.
Any stories about unsuccessful attempts using a z drag?
Yes, once was enough.
Any injuries from failed mechanical advantage systems?
Other interesting information from using mechanical advantage in rescue.
I try to never exceed 12:1 MA. To me, that means no more than 4 people pulling on a 3:1. If you're pulling too hard you're probably pulling the wrong direction. Keep it simple and safe.
Things learned from your experience.
A good swiftwater rescue course is invaluable. Once you've acquired that basic skillset, practice, practice, practice. Training + Practice + Experience and Good Judgement = Successful Rescues.
Feel free to shoot me an email if you want more details or specifics.