I've trained four children thus far. The 19 month old I trained started holding it early on and then going in the middle of the night. So I had to change her bowel movement schedule. I started giving her a teaspoon of flax seed in a cup of yogurt and a handful of shredded wheat first thing in the morning. After two days she couldn't hold it anymore. She went on the potty for two days and never held it anymore.
Another child I trained knew I was going to get the panties to put on so she went in her pull-up before I could get back. She would always cry for a pull-up at home and then go hide in another room. So I took her in the bathroom, told her she was too big for me to clean her bottom, and she would have to clean herself. She thought I was joking and said it was gross. I told her yes it is gross and I'm not doing it for you. I helped her get her pull=up off and in a plastic bag. I then helped her wipe and showed her how disgusting it was. I kept telling her if she went on the potty it wouldn't be so nasty. She could just poop, I would help her wipe and then we would flush it away. Easy as pie.
The next day she cried for a pull-up, but I wouldn't give her one and then finally I just made her sit on the potty. I hugged her while she sat there and kissed her face, but I would not let her up. I told her it would be fine and it would be so much better than pooping in a pull-up. When she finally went, we cheered to beat the band, and I gave her some chocolate chips. She cried a little the next day too, but after that she was fine and has gone on the potty ever since.
I don't give the kids I train an option. They will go on the potty.
I would start giving her heavy fiber foods real early in the morning for a day or two and see if her bowel habits change. Then after two days or so keep her up from her nap...just skip it if you have to so that you can catch her needing to go poop. If you have to follow her around with the potty chair (or remind her every 10 minutes that she needs to go on the potty and she is not allowed to go in her panties) She will have to go after all the fiber for two days. Keep that up until she's comfortable going on the potty. It will change her schedule too.
Good luck!! Poopies on the potty are usually more of a challeng, but it can be done. Sometimes they fight it, but they get over it and they won't be traumatized if you force the issue a little. Lots of hugs and kisses and calm voices...but insist that they go on the potty. No matter what they do, you can outwit them. During my mom's generation they spanked their children when they gave them a hard time...my uncle got a few in his day for pooping in his pants. He stopped pooping in his pants within the first week and he's fine to this day...he never suffered any trauma.
So I think insisting that she go on the potty with a little help from some fiber will be fine. Don't allow her to go in her panties and if she does...make her help you clean herself up. Cheer like a mad woman if she goes on the potty and maybe even give her a chocolate chip or two.