I would think that when you add the brown sugar at packaging time (if you stop fermentation) the beer would turn out too sweet. I'd put it in the boil for the last 15 minutes. I made a Brazilian lager using brown sugar and it turned out great. I wouldn't use maple syrup in a batch again. I made a maple syrup beer a few years ago and added the syrup in the boil (15 minutes) and it turned out nasty. If I could offer any suggestions, add the maple syrup for packaging, not in the boil.