With good grade B maple, you're probably at less fermentable sugars in it (compared with honey), I would add it once the wort is cooling down since you'll get different flavors it you add it hot compared with adding it at a lower temp. I think that once you've passed about 180F it should be good. You could just get a quart and add half during the cool-down and then some once fermentation has slowed, and the rest into keg before chilling and force carbonating it. See how it comes out that way and adjust for next time.
BTW, I have 10 quarts of grade B syrup on hand. :rockin: Used more in a 14% maple mead I made last December (going to bottle this month). Trying to decide what strength to make the second batch.
Of course, I'm NOT buying grade A syrup ever again. This stuff is GREAT on pancakes, waffles, etc... Kicks grade A's ass all over the playground.