I think the final product needs to have sugar in it (the commercial shandy), and this won't happen if the yeast is still active. I don't think you are going to be able to naturally carb this unless you plan on adding the lemonade after, which will cut your carbonation in half, and you will still need to kill the yeast. Yes I do suppose you could use sugar free lemonade and it would probably come out "ok". That is probably the only way to make this without using sugar. You don't want your sugar to ferment out as you won't get the proper flavor.
I do plan on making this very soon to have for the summer. My plan was to make a very light Belgian wheat to completion (carb'd and fermented), then experiment with lemonade levels to see what works. When I get the right taste, I'll kill the yeast, make the mix and recarb the total beer in the kegerator. I'll put this at a high carbonation level like 15 psi.
I was planning on making homemade lemonade with lemons and real sugar and NOT to use any store bought stuff.