This sounds delicious. Rogue hazelnut brown is one of my favorite beers, and nut browns in general are I think my favorite style. Never thought of a nut porter though, I may have to experiment with that as well!
Hazelnut extract does exist, here's a pic of a bottle I have of brewcraft hazelnut extract. Bottle says use up to 2oz for 5 gallons. This bottle came with a brewcraft extract kit for rogue hazelnut brown that I received for Christmas but havnt brewed yet. I tasted it just now and it tastes exactly like the hazelnut flavor of the rogue beer. Good thing I pulled it out to take a pic of it for you, the label says to keep it refrigerated. Oops.