The Feingold Association (www.feingold.org) claims on their web-site that the Feingold Program can help with eczema. Basically the program is an elimination diet which excludes the following: artificial colors (dye), artificial flavors, the preservatives BHA, BHT and TBHQ, artificial sweeteners and synthetic fragrances from foods, personal care and household products. Some foods are also eliminated temporarily and are reintroduced to determine whether they are a problem. The above additives are never reintroduced.
You don't say what you eliminated from her diet, but you may want to look into whether Feingold eliminates some things which you have not already tried. Also, processed foods (and even some not so processed foods such as low fat milk) can contain hidden additives not listed on labels. These are almost impossible to eliminate without contacting manufacturers (or joining Feingold which has already contacted them and publishes a book of additive free foods).
Unfortunately, there is no "test" to see if Feingold will help or not - you just have to try it and see. Also, they go into other areas of dietary exclusion such as gluten, dairy and additional foods/additives for extremely sensitive individuals (sounds like your daughter may be one of those). A clue that Feingold might help in this case is the age it started - around the time she probably started eating some solids.