Do you think you'd get a low pH from the lemonade that would adversely affect the yeast?
Probably not. I've used wine yeasts to ferment pre-soured wort to make sour beers without issue. Wine yeast is pretty hardy.
I should add, I've made 100% fermented grapefruit juice, and it was pretty undrinkable by itself, so I wouldn't ferment 100% lemon juice. Diluted in the ~50% range the fermented grapefruit juice was pretty good.