Maple Oatmeal Stout

Disclaimer: never personally used Maple in my beers

Of the Maple homebrews and pro beers I've had, the ones where maple was used at bottling (to prime the bottles instead of using sugar) were the ones with the most pronounced maple flavor. Rock Art made a maple wheat beer but it was flat, I emailed the guy incase a bad batch made it out of the brewery and he said it was due to adding it late in the boil and in the future they'll rethink when to add it. But as to how much to use, I can't help you there, sorry.

 

Maple Syrup will not make your beer sweet. The sugars in it are fully fermentable. I'd be more concerned about the beer being too dry, if too much maple syrup is substituted for malt grains/extracts, whose sugars/dextrins are not fully fermentable.

 

My advice about using honey, syrups, etc. for bottle priming is usually... don't, unless you are doing it for fun. For two reasons...
1) The amount needed for priming is pretty small, so there's not going to be much flavor contribution as compared to using a pound or two in the boil or fermenter.
2) Unless you know the sugar content of your particular syrup, your carbonation level is going to be a crapshoot. I wouldn't rely on any assumptions any online calculators make about the sugar content.

 

I would recommend adding very late in the boil. Or even to the fermenter at high krausen. Again, unless you know the actual sugar content of your syrup and are comfortable with the calculations, I wouldn't recommend priming with it at all.