My 4 1/2 yr old DD was uninterested in the loo for pooing, regularly doing it in her pants, immune to all motivational things, or anything else such as being told off or reasoned with. I have been following the bowel problems thread... also looking for solutions here. Anyway I came up with this and so far it has worked!
I told DD the next time she pooed in pants I would send her favourite toy back to Santa (important it goes back to Santa to remove any hope of looking for it!). I followed through - took away major toy. Very traumatic, but I stuck with it. Then we made a star chart together - 4 stars and I ring Santa up for the toy back. She gets a star for getting the poo in the loo - any poo in pants and another toy gets sent to Santa.
This worked for her almost immediately. She was so excited to be able to get the toy back. We made a big thing about ringing Santa and having it appear. She also gets a small treat for getting a poo in, as well as a sticker. It stopped her withholding as she wanted a sticker so badly.
She tried it on a bit by getting most of the poo in the loo and a bit in her pants. I let this go once, then sent a toy to Santa. So far (this is one week) we have had all poos in loo and she is really proud of herself and all the stickers she's won.
Will keep you posted, but hope this might work for others! Costs nothing and seems to create major motivation!