I was at the Cookbook Co. and we made a version of Tom Yum soup. The chef went off the recipe and added a japanese ingredient. It was a highly concentrated lemon/orange flavouring. He added about 2 tablespoons for a very large pot of soup and it added a lot of citrus flavour to the soup.
Not what you're looking for but I had a bunch of lemons that would have gone bad so I froze them. I grind them into soups, stews, etc., peels and all. It makes soups awesome.