IMO a not a bad idea, but I wouldn't go with a diet shake - hear me out.
Rather than a "slim fast" with raw veggies, why not create yourself a green smoothie with 6-10 servings of vegetables, a serving of healthy flaxseed oil, and a scoop of protein powder (30g protein).
That way, you can have your shake but instead of a junky diet shake, have an enourmous amount of vitamins, healthy fats, and protein.
I've drank this one myself and LOVE it. But I add protein powder to make it a meal.
http://www.bodyrock.tv/2011/03/28/green-smoothie/