Oy, I hear you. We've tried corn syrup, mineral oil, prunes, prune juice, and glycerine suppositories. Everything would work for a short time and then have no effect. We finally broke down and moved onto MiraLax. It's basically the only thing that will allow her to go semi-regularly.
Once she's totally potty trained (she's still hesitant to poop on the potty,) we'll try backing her off and letting her try it naturally. I just don't want painful poop issues to derail all of her hard PT work.