DD had a similar problem except that she would say she was scared to poo in the potty or toilet. She claimed it would hurt, but was fine to do it in her nappy. I had just got a Bounty bag from the hospital we went to book for delivery of DS, and i pulled out the sample tub of Heinz sudocream. I would tell her it was her "special bottom cream". I'd put her on the toilet, put on a tiny bit of the sudocream while explaining to her in a "fun" voice that it would make sure it doesn't hurt. And she started to go.
One day we went shopping. she said she had to do a poo. She asked for the "Special bottom cream" and i panicked. I told her i didn't have it in the nappy bag. S eh jumped off and went through the bag and found it. Apparently she packed it before we left the house. After a while she grew out of asking for the sudocream and would just do it in the toilet.
Shame about the regression on the whole toilet thing after DS arrived. *sigh*
Life has been hell for a few months now. Just can't get her to stop wetting herself.. though she doens't poo in her underwear thank goodness!