My son (now almost 4) was the same way last summer - we tried to be as positive as we could, and gave him a goal that if he went poop in the potty for a week, he could get a special toy. Unfortunately, that didn't work!
I totally agree with the "commando" method - when I knew he was about to go, I'd just take his pants off and let him run around the house - he'd stop dead in his tracks when the urge hit him, then high-tailed it right to the potty! Once he had done it a few times, if he continued to go in his pants despite knowing it was time to go to the potty, we then started taking away a favorite toy or lovey blanket each time he pooped in his pants (putting the toy in "time out"), and would give it back the next time he pooped in the potty. That did the trick for us! When all of his toys were out of "time out", and he was accident-free for a week, he did get his special toy.