I haven't got daughters, but if I did have, I guess I would have had to tackle this situation at some point. Having had three boys, it is far easier, because they (or you) can point their willy so as not to wee all over their shoes.
I can understand your objection to the shop doorway (though, as others have said, desperate times do sometimes call for desperate measures), and I do agree with you that it was not good for the little girl's genitals to be so visible (though again, I don't know all the circumstances, so it may have been the only option). All that said, in an emergency, you do what you have to do in order to prevent your child from wetting themselves - something that they could equally well remember and be embarrassed by.
I do remember dh coming home with the boys after a day in London, and telling me that ds1, who was fairly recently potty trained but pretty reliable, had suddenly announced on the underground that he needed a wee NOW - there was no way he was going to last 'til the next station, let alone until dh got them all off the train and them and the buggy up the stairs to the nearest loo, so he grabbed one of ds2's nappies out of the bag, whipped ds1's trousers down, put the nappy in position, hauled the trousers back up and let ds1 do what he needed to do. It may have raised the odd eyebrow - but was surely less offensive than ds1 wetting himself all over the train.
I also remember trying to wee al fresco when I was a small girl, and all I ever achieved was a wet leg and a welly full of wee!