I've never heard of them being added in boil, but I'd be interested to see how it turns out. Most of the time, I believe it's added to secondary. Otherwise, adding it in primary might contribute to off flavors from the Yeast consuming the sugars. No idea if this is actually true, just a thought I had. I added chili pepper cacao nibs to my porter, and it's bottle conditioning now, but not in the boil.