R.M.
I have two thoughts, based on our pottying experience. This will take time, but try to move her pooping time to either the morning, or the evening. If she lets you, just set her down on the potty every morning / evening, tell her you are waiting for the poop to come out. You can read or play some games. Wait half an hour at least (if you have that kind of time). Over time you can get her on a morning or evening routine, so the naps won't be affected. The other suggestion is to give her time to poop before the nap, but then if it happens during the nap, just leave her with it. I know it's not ideal, but it might help her connect the dots.