I don't see why the diet soda mix wouldn't work, but perhaps more experienced bakers will weigh in with their opinions contrary to mine. Since they're so thin, I would start them at 15 minutes and check them every couple of minutes after that if they're not done. Do you have a Cake Tester? If not, use a toothpick. It should come out clean. They should freeze rather well, but I'd either put wax paper between them or freeze them individually on a cookie sheet and then bag them. As for fiber...not sure. My first thought was Metamucil (or something similar) or flaxseed, but I'm not sure. I know the fiber mixes come flavorless and some say you can mix them in water without tasting them so that would work, but I have NO idea how that would work in a cake... Hope this helps!