I did this once. I was told to use clear drinks like diet 7-Up or diet Sprite with yellow or white cake mixes and the darker ones like diet Coke or diet Pepsi with chocolate. I also added 2 egg whites, which by the time you divide it them by how many servings you slice your cake into that doesn't add any more points.
I just figured out the points for the cake mix based on how many servings it makes and used that. I think the one I made was 3 pts. for 1/12 of the cake. I didn't put any frosting or topping of any kind and it was WONDERFUL.
My SIL made it once for all of us and she spread fat free/sugar free lemon pudding on the top like frosting. Can't remember the points for that but it was very low and tasted great! Ummm, makes me want to make it now!!!