Oh this was my world for about a year... my daughter started doing this at 2 and right before age 3 we finally stopped the Miralax and she started going on the potty. I HATED the Miralax too for exactly the same reason - it's not addressing the problem, only aggravating the control issue. It is a control issue. Period. I don't know why they fixate on this, and it is so frustrating!
We even had to visit the ER after she didn't go for 10 days (and was obviously in pain) and while in the waiting room she went...now dubbed the $150 poop.
So, we talked about it constantly with her. Now, we still talk about pooping and size and when did you go, etc. Also, we totally removed all cheese, milk, and fried foods from her diet. It helped that Dad was diagnosed with diverticulitis about the same time, so we really "fiber'ed" up.
We only used about 1/2 tsp with my daughter (the ped recommended 1 tsp). I felt that if we could get her to not hurt when she went, but not lose control, it would be better, and it was. Really, it was a big fight for awhile. I'd catch her clenching over behind the couch and pick her up and run into the bathroom and sit her on the potty. She'd cry, I'd cry. But it taught her that going was OK and that the potty was a safe place where she had privacy. Also, I did NOT make a big deal about it with cheering, etc. as this tended to aggravate the control issue. I'd just calmly go in there and wipe her and say "good job, does your tummy feel better?" and things would go back to normal.
I wish I could offer you more, but you certainly have my moral support! At least I can say that it is a phase, and "this too shall pass". Now, she's totally cool about it and will even go at the grocery store.