I used oak chips soaked in Bourbon in a Blond Ale. Not knowing any better, I used a pound of oak and soaked it in about 12oz. of Bourbon, strained it and put it into the secondary for a week. I must say, it was drinkable, but not good by any stretch, way too overpowering. I think your 3oz. ingredient should work fine along with the brandy. However, doesnt a Schwarzbier a use smoked malts? I’m wondering how the brandy and oak will work with a smoked beer.