Hmmm yes I remember the poo pants.....I was tempted to cut them off and throw them away sometimes - gross!
Eliza didn't do it much but it is very common for kids to wee in the toilet and refuse to poo in it.
Does rewarding her work? If you find her "currency" a treat, activity, toy whatever you could try. I did this briefly with Eliza at first and by 2 weeks we had stopped the reward and she was TT.
Otherwise side snapping undies? LOL!
Hugs Trish