We're right there with you, except we also have the chronic constipation issue (DD is on a daily dose on Miralax). She asks for a pull-up to poop, or waits until she's in her nighttime pull-up to do so (she's not night trained yet). She's on a nighttime pooping schedule, so it is hard to avoid the wait-until-bedtime routine. I keep trying to tell her that nighttime pull ups are more expensive! LOL. I'm so afraid of withholding, which is totally something she would do, that we are moving very slowly. Just last week I got her to sit on the potty and "try" to poop while she played with a special poop-on-the-potty toy. We only got as far as "try".
Mom to a spirited, red-headed, former 28-weeker 10/2009 and a more mellow monkey 12/2013.