On page 10 of the WSM Owners Manual, Weber suggests adding "bay leaves, garlic cloves, mint leaves, orange or lemon peel, and various spices to the charcoal for added flavor." Whole onions are also popular among many folks.
Why not give it a try, then come back and give us a full report?
Garlic and onion create an incredible flavor complement especially to hickory...would tend to think that rosemary, dill, chive, etc...would do the same if not more.