I just used hickory syrup in a beer, and I added it just as fermentation was beginning to slow. Fermentation took off again, of course, but plenty of hickory flavor remained when activity subsided. I'm very pleased with the outcome. I used 16 fl. oz. of syrup in 5 gallons of session-strength beer.
hickory syrup is a really different beast than maple though isn't it? If I am remembering correctly hickory syrup is made by boiling the bark of the hickory in sugar syrup rather than concentrating the sap.
I've used a cup in addition to dextrose for priming a Nut Brown Ale and got a nice maple flavor.
This is an interesting point on the bottling sugar addition with maple. I did the calculations at one point and to prime just with maple takes close to a lb of syrup even for a middle of the road carb level. It's only about 70% sugar (>=66%). imagine 12$ just for priming sugar! I have wanted to make a maple wine for years but the cost is too high so far for me to pull the trigger.
i once used it as a priming sugar.
it turned out okay, but the maple flavor was not intense and dissipated very quickly.
trader joe's now sells maple sugar and i'm curious about that for priming. i know adding flavors that late in isn't ideal but that's part of the fun about brewing for me.
I think that you would get less flavour using the maple sugar (dry) because they start out with such a light grade of syrup so that it doesn't get dark. grade B dark amber, or bakers grade if you can get it, is your best bet for flavour.
you could also add some fenugreek as that is what is used to make aritificial maple flavour but I have no idea how much or when you would do that.