I'm not sure, but my instinct says that too much of any one thing might not be a good idea whilst breastfeeding. So whilst a shake instead of lunch might be ok, I wouldn't swap more that one meal for a shake.

If you can't find out for certain, how about a home made veggie soup for lunch instead? That way you know it will be low calorie, salt, etc, whilst not having to worry about any hidden ingredients. I make a huge pot of soup and freeze it in portions, so I can always have one for lunch and the fibre/liquid mix is also great for the old digestive system.