Barley is indeed on plan for Trim Healthy Mamas. Hulled barley is preferred over pearled barley. On the Trim Healthy Mama plan you can have up to ¾ cup of barley as your fuel for an E meal. If the barley in this dessert is your only carbs for your meal, you'll need at least a half cup as your E fuel. If you're eating this along with another carb source in an E meal, make sure that you keep your portions moderate so you don't exceed your 45 gram carb limit.
Feel free to add some pureed fruit to this "tapioca". Chunks of fruit would be great in this as well. I've eaten the vanilla version with some sugar-free maple-flavored syrup, which was great.