My suggestion would be to reward her with a big prize when he goes poop in the potty. I would not consider it a bribe so much as a reward for a job well done. A bribe you would give before she goes, hoping it will make her want to, a reward comes after the task is complete. I think the reward needs to be immediate (not a future trip to Disney,or a promise of picking a toy at Wal-Mart) and special (not the same old little treats she sees all the time).
Here's how it went for me: My son was competely trained on pee-pee by 2 1/2, but our final hurdle was getting him to go poop in the potty. He just wouldn't do it, no matter how much I pleaded with him. Similar to your situation, he only wore pull-ups at night, so he would either wait until he had the pull-up on and go poop at night or he would hold it as long as he could, and then he would ask me to put a pull-up on. I would let him, becuase I wanted him to go, not hold it. Then one day, I finally refused to get a pull-up and told him he HAD to go in the potty no matter what. Instead, he figured out how to get one of his little brother's diapers out of the drawer and diapered himself!
Candies and stickers and such did not work for us either. He needed more motivation. So, we bought him a special toy (nothing outrageous - a toy train for about $25, but big enough that it is something he would normally only get for Christmas or birthday otherwise, so he knew this was a special occasion). We showed it to him in the evening (around the time he would normally ask for a pull-up). He got very excited, and then I explained that we would only open the toy if he went poop in the potty. He decided he would try, but he didn't go (I think he thought just trying would get him the toy) so, no toy. We put it away in the closet unopened and told him that we could try again tomorrow. He was determined though, and tried again, and sat on that potty until he went. We sang the potty song (something I just made up), did the potty dance, gave him the toy and made a big deal out of it.
At bed time, we put the toy away, and told him that it was special and he could only play with it after going poop in the potty. So, the next night he was excited to go poop in the potty and get to play with his toy again. He liked the idea that it was special that way. Also, I made sure that his little brother was not allowed to play with it, so that made it even more special to him. He went on for about a week and eventually stopped asking for the toy after every poop, but continued to be fully 100% potty trained since then and has not once asked for a pull-up. He can play with the toy all the time now, and is proud of himself and even tells everyone "I'm potty trained!" Honestly, I think they just have to get used to it for a while, and then they make the connection that it's what they have to do.
Also, once he had pooped in the potty, I eliminated diapers and pull-ups altogether, even at night, so as not to even give him the option of falling back on them. My husband thought he should have pull-up at night until age 4, but he has done just fine. Over three months now, and no accidents.
I also really like the idea the other mom had about cutting a hole in the pull-up. That's a great idea. You may want to try that first. Good luck!