Will Campden disinfect flavoring additions in a primary?

Campden doesn't kill cultured yeast, at least not at the levels you'd want to use it at without significantly hindering the flavor and aroma of your beer. The problem with using Campden (Potassium Metabisulfite) is that it adds significant levels of free and bound SO₂ to your beer. This will cause your beer to smell and taste like sulfur. Wine is typically kept at about 50 ppm of free SO₂, as this is high enough to kill wild yeast and spoilage organisms, but low enough to be below most people's detection threshold. This level isn't high enough to kill cultured yeast. When you add Campden to your cider, you're protecting it from spoilage while still allowing your cultured yeast to thrive.

When winemakers want to stop their yeast before it ferments dry, they sulfite to about 50ppm and add Potassium Sorbate. It's this combination that prevents the yeast from reproducing and causes them to go dormant. It's a balancing act for winemakers. Not enough and the yeast will come back to life and start fermenting the sugars again. Too much and the wine will taste like eggs, rubber, cabbage, etc.

Additionally, Campden isn't a very effective sanitizer compared to Star San, Iodophor, or Alcohol. Campden works reasonably well in wine because ABV levels are typically above 10%, often in the 13% range. In beer, they're usually in the 5% range, give or take a few. Wine typically has a pH much lower than beer as well. At beer's low abv and relatively high pH levels, sulphites aren't the best sanitizer.

For those reasons, it's best not to use Campden to sanitize your flavorants - mainly because of the risk of off flavors, but also because it's not the best sanitizer. I recommend sanitizing your beans and nibs with strong alcohol such as vodka or whiskey (if the whiskey's flavor will play well with your beer).