H.D.
For us, we used a portable DVD player (they didn't have iPad's 5 years ago) in the bathroom as a reinforcer. As soon as we'd see our son start to have "the face" we'd grab him and run to the toilet as fast as we could and rip his pants off. Majority of the time we made it, others, we had messes to clean up:( We told him if he'd poop on the potty he'd get to watch a show. Then, we'd only allow him to watch a few minutes at a time and we told him as soon as he wanted to go poop again he could watch more of his show. It seemed to keep him motivated as soon as he caught on. We also chose a show he was obsessed with and knew would keep him wanting more. I understand your frustration though, changing a big boys diaper isn't pleasant:( I should ask though, do they refuse so bad that they end up holding it in and cause encopresis?? Are their diapers more diarrhea then solid?