You can actually get a balanced daily diet out of mcdonalds if you ask for unsalted fries and get one of their salads once in a while.
Honestly sodium aside the only thing you need to do not to over eat at MC is to get water instead of soda and just regular sized burger.
It will be around 600-700 cals per meal with fries which means you can eat 3 of those a day and maintain weight or even lose some depending on your age, sex, bodymass and daily regiment.
I did this myself around the same time as Super Size Me for a handful of reasons, none of which were to prove a point. I had good results from it. Would I have been even better off if I wasn't eating fast food for every meal? Probably. But I certainly wasn't worse off for eating that way and it scratched the itches which led me to that path in the first place.