Hard time getting a response, I see. I'll give it a shot for ya. I don't claim to be an expert on this particular subject but I did stay at a holiday in last night. I also brewed a hazelnut brown recently. From what I'm told, if you just put the nuts in secondary, the oils will kill your head retention. I ended up making my own hazelnut extract. I'm not sure if the same process would work for pistachios or not, but here's what I did:
Roast the nuts at 325 for about ten minutes. I then removed the skins by rubbing all of them with a dish towel. I finely chopped all of the nuts and put them in a mason jar and filled the jar with vodka a little above the top of the nuts. I let this mixture sit for about a month. I then strained as much of the vodka from the nuts as possible into another mason jar. Then in a small saucepan, I boiled a mixture of the crushed nuts, 1/2 cup sugar, and 1/4 cup water for a couple of minutes. I let the nuts steep in this this syrup for a few more minutes and then strained the syrup into the container with the vodka.
That was pretty much it. It took some time, but it came out smelling delicious. Then I simply added a bunch of it when I kegged the beer. I think I added about 5 or 6 TBSP, but I'm not sure if this was a good amount since I haven't tasted it yet. Anyway, I hope this is somewhat helpful. Good luck.