I don't have any recommendations for amount, but I will say do it! A local brewery called the Hideout did a hazelnut pale ale a couple months ago. It was very delicious.
After doing a little searching on here, it looks like for a 5 gallon batch, most people use 1/2 to 3/4 lb of hazelnut added to the mash. The oils from the hazelnuts will kill head retention though, if that is important to you.
I've had the hideout's and it's ridiculously good, I think they artificial flavor it tho, I'm going to use real hazelnut. Mash tun idea sound on point. Thanks!
Founders had a Frangelic Porter on tap for awhile. I wondered why Frangelic, then I found Frangelico. I wonder if they didn't use that? http://frangelico.com/