I would add it in the secondary - too much of it will ferment out otherwise, leaving very little maple flavor.
SO I am planning a maple barley wine and I was thinking about this when to add issue. When maple syrup is made it is boiled for a LONG time. so I don't imagine more boilding would matter at all. I also wonder if you would lose flavor/aroma to CO2. I don't know just wondering. Given that any remainng flavors are obviously pretty stable would you lose anything to the scrubbing effects of CO2?
I wasn't concerned about the boil so much as the sugars fermenting out, like honey. If you add it early, you are essentially giving the beer a gravity boost, but run the risk of not retaining much of the flavor as it ferments out.
I have a couple questions pertaining to adding it to the secondary:
Will I need to "Pasteurize the maple syrup like I would honey? When I made a honey cream ale, I added the honey at flame out and let it sit for about 10 minutes before chilling the wort. That kind of pasteurized the honey. And since honey is not 100% fermentable I still retained some honey flavor and sweetness.
Will it start a secondary fermentation because maple syrup is 100% fermentable sugar?
hmm, if pasteurizing makes you feel more comfortable, I say go for t it. If you are bottling (not kegging and force carbing), you may want to add late in the boil or sometime in primary, so that you don't create bottle bombs. I've primed with maple syrup and still had some hint of maple.
Having said all that, the only maple beer I've enjoyed was one made that substituted maple sap for water (& primed with syrup). Commercial exapmles have been way too cloyingly sweet to me and ended up being poured down the drain.