H.P.
Mine holds his in, too. He holds it and holds it until what's in there is just a brick. It's been a while since he's had quite a brick, though. What works for him is Miralax, long walks, and juiced pears and apples. He is mostly gluten free right now, just because of the issues that I have with gluten--pain, constipation, stiff joints, lethargy...--until he is a little older.
I would say to continue to do whatever you do to treat her little bottom and make boo-time fun. My son has a bouncey activity center that we can still put him in when he's got to go. He jumps around and lets it out, and we praise him for it. Super warm baths with a little (doctor-prescribed) epsom salts might help, too.
(Fyi, there is a company that specializes in harvesting gluten-free oats, if that interests you at all. I've tested negative for Celiac, but I have a gluten sensitivity and really missed eating oatmeal. You can find them online at http://www.glutenfreeoats.com/.)