What about if you try to "undo" it the way you think you might have encouraged it to happen ? When you go to change her diaper, say something stupid like, "Is there a poop in here?" very excitedly, and say, "I'm going on a poop hunt." etc. And then if you find even a little bit, get excited over it.
When we were potty training my 1st daughter, at 2 1/2, she was great except for pooping. She wouldn't do it on the toilet. She'd hold it in, then go in a corner and push it out. It was obvious what she was doing, so one day when I caught her, I said to my spouse, "She's pooping. Quick! Grab her and put her on the toilet" He swooped in, picked her up, and sat her on the toilet, then made a big deal about the fact that she did it on the toilet. I think they even waved good-bye when it swirled out of view. I don't have any idea what was going on in her head, whether there was something scary about pooping in the toilet, vs in the pants, ?? , but after that, she never failed to do it in the toilet.
The other thing you can do is to sit with her at a specific time each day (like after breakfast or something) with her on the potty chair and read books for 1/2 hour. If a poop comes out, YAHOO !!!! If not, it was a good mom/daughter time. It might work best if her little bro is napping, so you don't have to worry about what he's doing while you're having a fun time with her.
I would be inclined not to worry about her training pants getting brown and/or hard to clean. You can chlorox them the way you would cloth diapers before washing them, and when she's got it down, you can buy new ones. Just don't make a big deal about grubby undies. They are washable and replaceable. And if she poops in them, you can tell her it's always better to have the poop out rather than in, and do you think if we read a book or two you could get some more out ?
is there any special kind of candy she likes ? (I can hear all the moms booing over this one !) If so, you can give her a small piece of it every time she gets a poop out. Anything to make it a positive experience. With all the stuff out there about sugar these days, I want to say that my two older girls were brought up on healthy foods, and lots of enforced exercise, cuz I exercised a lot and dieted a lot when I was younger. They are both overweight today. My younger 2, born 10 years later, are as skinny as they come, and eat sugared cereals and ice cream. Go figure. I think they don't crave it because we didn't withhold it. So I'm not into the "avoid sugar" thing anymore. Use it to your advantage. Give her one M&M every time even a little piece of poop comes out (even if it's in her pants, at first). Frankly, the lecithin in the M&M's may help move them through. :-) Either way, it's a pretty reward, and that little piece of candy at a time isn't going to cause diabetis or sugar highs. And if she gets the log out -- why not give her 5 or 10, she worked for them !
It's a little bribery, it's a little behaviorism, it's a little bit fun for her. And if it works, it's a whole lot less frustration for you (and less guilt, cuz right now you are blaming yourself).
Good luck !