Interesting idea! I don't know what would happen if you soaked in cold coffee, but people certainly do this with wine - and I think wine is acidic, isn't it? I suppose another option would be to do an espresso grind, boil the grounds in some water (to sterilize), and cool - then mix the coffee AND grounds with the curds, or add the coffee to the milk before making, and add the grounds to the curds - this is how many people handle adding herbs.