Interesting idea. I've done oak barrel chips soaked in bourbon, but never tried for cocoa nibs. I'd add it after fermentation is complete. How much are you planning to put in?
Did a fresh batch. Diluted as per recommendation (10ml/5L). Ph dropped to 2.6 without much mixing. Used just filtered tap water Ph 6.9. Got 500ml of Chemsan about 9 month old.