DO NOT USE Muratic acid on nickel covers....
Muratic Acid is too strong for nickel, it'll probably ruin your cover.
To your question, you can age nickel plating with vinegar. Take a rag and soak it with vinegar... put that and the nickel cover (not the pickup itself, only the cover) in a sealed plastic container or plastic bag. So that the fumes of the vinegar can react with the nickel plating without letting fresh air in. It will slowly eat the plating and dull it nicely. You have to clean the cover with naptha or some alcohol first so you don't have any fingermarks or anything on it, otherwise it will be awfully "finger printed" afterwards. You might need to have it in the sealed container/bag for a few hours before you see any result.