I just brewed a stout a couple of weeks ago and sectioned off a half gallon to try this same thing out. I added 1g cinnamon stick, 1g allspice berries, .5g ground nutmeg for the half gallon. I just stole a sample and it has an intense aroma from the nutmeg and I also get hints of the allspice but nothing really from the cinnamon. Nutmeg seeds might tone it down a bit but if you were to use ground i would hold back a bit.
Not sure about the nutmeg, but I brewed an oatmeal/java stout and threw in one cinnamon stick at flameout and let it go into the primary. It was a five gallon batch, but three weeks after bottling I feel like I can detect it. That said, it's subtle.