Whirlfloc is irish moss blended with carrageenan. Not quite the same. I think the question comes down to is it close enough and does the carrageenan addition help all that much. I personally can't tell if it does or not, but that's just me
Everything I've read has pointed at flavorings to be the only way to get a true "chocolatey" flavor as nibs have been known to make it go super bitter and give a dark chocolate flavor more than anything else. So, I'd go the flavoring route for that note and then use some lactose and vanilla bean...