Not really useful advice - but my DS is not quite 3 and a half and we only started making any real progress when he started wearing his sisters clothes! We have 100% success rate when wearing his sisters stuff (whether at home or daycare) but only a 30% success rate with poos when wearing his own. I don't think is coincidence as I had noticed it but not said anything to daycare and then they mentioned it to me. I let him choose which clothes in the morning and if he chooses hers we don't have any issues. My only theory is he 'feels' like an older child when he is wearing her stuff (he does look up to her alot) and that reminds him to use the toilet.

Kids are just plain weird sometimes :-)
I just went with the wearing his sisters clothes and then one day he did a poo at daycare when he had chosen to wear his own clothes and we haven't really looked back since. He doesn't wear her clothes so much now (he has his own tinkerbell undies now) and have only had the odd accident in about 6 weeks and always when not at home and he wasn't able to find me.