Here is a link to a recipe calculator that I use to generate the US food label for my freelance recipes.
http://recipes.sparkpeople.com/recipe-calculator.asp
Using it with this link to adjust the amount of residual carbs should give you a decent estimate:
http://www.dd26943.com/davesdreaded/tools/convert.htm
Note that the proteins are going to be fairly soluble in the alcohol so you can reasonably assume total extraction. The fats are another issue, but probably minimal. I'd ignore any residual yeast, if you clarify your brew, and fiber from the grains/fruit (hard to estimate soluble fiber).
So, here is an example. I brewed a batch of pear cider. Putting in the raw ingredients (58 pounds of pears, 1 oz yeast for 53 12 oz servings), I get the following nutritional facts from the recipe calculator:
Nutrition Facts
User Entered Recipe
53 Servings
Amount Per Serving
Calories 261.1
Total Fat 1.8 g
Saturated Fat 0.1 g
Polyunsaturated Fat 0.4 g
Monounsaturated Fat 0.4 g
Cholesterol 0.0 mg
Sodium 0.2 mg
Potassium 555.3 mg
Total Carbohydrate 66.8 g
Dietary Fiber 10.6 g
Sugars 0.0 g
Protein 1.8 g
Vitamin A 1.8 %
Vitamin B-12 0.0 %
Vitamin B-6 4.0 %
Vitamin C 29.4 %
Vitamin D 0.0 %
Vitamin E 11.0 %
Calcium 4.9 %
Copper 25.0 %
Folate 7.7 %
Iron 6.1 %
Magnesium 6.6 %
Manganese 16.8 %
Niacin 2.2 %
Pantothenic Acid 3.1 %
Phosphorus 4.9 %
Riboflavin 10.4 %
Selenium 6.3 %
Thiamin 5.9 %
Zinc 3.5 %
Since I juiced the pears, I remove the fiber data (not a lot of fiber dissolves). I remove the fat data (not very soluble) and the carbohydrate calorie data, substituting the data from the Dave's calculator (note that in my case, there is very little sucrose - thus no sugars listed in nutrition label, the remaining carbs being the fructose that is converted to alcohol and in the SG calculations, so multiply the original carbs by the remaining "solids" after attenuation - see Dave's SG). So the adjusted label looks like this:
Nutrition Facts
User Entered Recipe
53 Servings (12 oz servings)
Amount Per Serving
Calories 187
Total Fat 0 g
Saturated Fat 0 g
Polyunsaturated Fat 0 g
Monounsaturated Fat 0 g
Cholesterol 0.0 mg
Sodium 0.2 mg
Potassium 555.3 mg
Total Carbohydrate 12.0 g
Dietary Fiber 0 g
Sugars 0.0 g
Protein 1.8 g
Vitamin A 1.8 %
Vitamin B-12 0.0 %
Vitamin B-6 4.0 %
Vitamin C 29.4 %
Vitamin D 0.0 %
Vitamin E 11.0 %
Calcium 4.9 %
Copper 25.0 %
Folate 7.7 %
Iron 6.1 %
Magnesium 6.6 %
Manganese 16.8 %
Niacin 2.2 %
Pantothenic Acid 3.1 %
Phosphorus 4.9 %
Riboflavin 10.4 %
Selenium 6.3 %
Thiamin 5.9 %
Zinc 3.5 %