I haven't added maple syrup to beer before, but I'll be adding molasses to a harvest ale soon. Forgive me if I'm mentioning something you already know, but adding extra fermentable sugars at bottling wouldn't only make it sweet, it could potentially over-carbonate your beer and create bottle bombs. You can only add as much fermentable sugar at bottling that can safely be consumed by residual yeast to create a pressurized environment. They won't stop eating just because your beer is finished carbonating. They'll keep eating till all the sugar is gone, which could potentially create too much pressure for the glass bottles to handle. Been there, done that. Not good. Big mess.
I'm going to add a pound of molasses at flame out, in hopes that any buggies get pasteurized. If you're concerned about there not being enough residual maple flavor, you just need to balance the recipe to favor more syrup. Yeast won't eat all the syrup. They'll leave some unfermentable sugar chains and flavors behind, but in order to be able to taste that maple clearly, you may need to favor it as part of your "grain bill." It's like adding strawberries. Yeast eat the sugar off strawberries, that sugar becomes alcohol, and what's left behind for the final product isn't very distinct in small quantities, so, the only answer is more strawberries. If your alcohol level is too high already to add as much maple syrup as you need to get past the flavor threshold, it's not like this batch of beer will be bad, just drink your delicious beer anyway, then adjust the ratio of malt:syrup on your next batch, based on your perception.
If you're feeling adventurous, add maple syrup to secondary, but it's still just going to ferment out. It's not super likely that your maple syrup has bacteria in it, but it's not impossible. If it did have bacteria, you'd be beginning a souring fermentation that might not likely end till you hit an FG of 1.000, and you almost definitely won't be able to taste the maple sweetness then, and the beer will be quite dry.
I'm kinda shooting from the hip here, so take all that with a grain of salt.
