Nibs in the mash seems fine. I wouldn't do the bar in the boil, that's adding a LOT of oil to the beer.
You could always steep some crushed nibs in vodka and if your beer isn't chocolatey enough at bottling, add some of the vodka. That's what chocolate extract is, you'd be making your own. I have some nibs and a piece of vanilla bean steeping in vodka just for flavoring lattes, works great.